why didn't the combatants copy each other's weapons

Reverse-engineering and then making copies of an enemy weapon can work, and can be much faster than designing from scratch. But it takes time - time to do the actual reverse engineering (i.e., the design work), and time to tool up for production. By the time that's all done, the enemy can have their next generation weapon systems in service - copying the enemy can easily result in always being behind technologically. For example, the Soviets acquired B-29s in mid-1944, and reverse engineered them, developing the Tupolev Tu-4. First flight was in mid-1947. Compared to the B-29 program, which began in response to USAAF specifications at the end of 1939, but also drew on design work which began in 1938, and led to first flight in late 1942, the Tu-4 had quicker development, but not that much quicker.

One question that has been asked before is

why not copy the improved T-34s?

First, consider the early T-34 - excellent mobility, and excellent armour. Germany first encountered the T-34 in June 1941. In November, it was decided that improved tanks were needed in response. It was suggested to simply copy the T-34, but copying the engine and making enough of them in time was not feasible. Therefore, some redesign was necessary. Of the two main competing designs, MAN's VK30.02(MAN) and Daimler-Benz's VK30.01(D) (after Henschel's VK30.01(H) was rejected as inadequately armed and Porsche pulled out), the VK30.01(D) was very T-34-like - easily different enough to not be a direct copy, but drawing on many elements of the T-34 design, as can be seen:

• https://commons.wikimedia.org/wiki/File:Comparison_of_VK_3002_proposals.jpg

(the top design is the MAN, and the bottom the Daimler-Benz). The MAN design was chosen for its better ergonomics, especially better turret ergonomics, and became the Panther. Also, the MAN turret could be produced quickly, since it was based on an existing design. The basic design work was finished in early 1942, which was only possible because the design drew on previous experience and design work on other tanks by MAN and Daimler Benz. A prototype with mild steel "armour" was ready in September 1942, and production began in early 1943. It is unlikely that a direct copy of the T-34 could have been in production so quickly, as tooling time would have been much greater. In the end, the German army got its improved tank faster, and as far as they were concerned, got a better tank, than if they copied the T-34.

Germany made two tank decisions in November 1941 in response to the T-34 (and KV tanks). First, as noted above, they planned an improved tank. Second, plans to up-gun the Panzer IV to a high-velocity 5cm gun were abandoned, and instead it was decided to up-gun it to a high-velocity 7.5cm gun. The up-gunned Panzer IV was in production in March 1942, beating the decision on the Panther design by a couple of months, and the introduction of the Panther into service by a long time. This gave the German army a tank fit to face the T-34, and made copying the T-34 (which would have taken a long time, due to tooling) rather pointless.

Similarly, when improved versions of the T-34 appeared, it would have taken too long to copy them for it to be useful. (The German army considered the T-34 a useful tank, and happily put captured tanks into service. If all that was required to produce them was to turn on a switch in a captured factory, it would have been done.)

As it was, the Panzer IV remained in production to the end of the war, despite the intent to replace it with the Panther. Factories were already making the Panzer IV, and could continue making them, without needing retooling to make Panthers. The time needed to retool those factories to produced Panthers (or Tiger I or Tiger II tanks) could not be afforded - tanks were needed immediately.

(The T-34 and KV tanks were not the first well-armoured tanks Germany had encountered - the Matilda II has earlier proved highly resistant to German tank guns and also anti-tank guns short of the 88mm.)

The air equivalent of the T-34 - that is, an aircraft that astounded the enemy when they first met it - was perhaps the Mitsubishi A6M Zero ("Zeke"). Captured examples were evaluated by the US and Britain, and they were impressed by the plane. However, by US and British standards, it provided insufficient protection for the pilot, and adding armour, self-sealing fuel tanks, etc. would have added weight and degraded its performance. It was not a good choice for copying. Instead, combat experience against the Zero, and analysis of captured aircraft, was fed into the design of the Grumman F6F Hellcat, which had been in progress as a successor to the F4F Wildcat since 1938, with a prototype ordered in mid-1941. Changes in design were made to make it a better Zero-killer. Since the design was already well in progress, this didn't add greatly to the time required for design and production. Of two options that the USN could have chosen in response to the Zero: (a) copying the Zero, or (b) making a Zero-killer, they chose (b) and got a better plane into service quicker.

As for copying rifles, the German army would have been happy to have copies of the Soviet semi-automatic SVT-40, if they were available in enough numbers and re-chambered for German ammunition. The SVT-40 was a good rifle, and the Red Army would also have been happy if they had enough of them. As it was, the Red Army was in the process of switching to it as it's standard rifle, but equipment losses in Barbarossa, and disruption of production due to the invasion led to the switch from the Moisin-Nagant to the SVT-40 being reversed. Germany's response to the SVT-40 was to design a new semi-automatic rifle, the Gewehr 43, combining design elements from its predecessor, the Gewehr 41, and the SVT-40. Under 500,000 were made, and the bolt-action Kar98k remained as the main rifle, with almost 15 million produced, simply due to being available in sufficient numbers.

These examples illustrate 3 reasons why direct copying wasn't done more often:

1. Modifying existing designs can be faster, due to taking advantage of existing design work and existing parts and production lines.

2. Copying an enemy weapon means that your new weapon will only be as good as the enemy's weapon. Designing a weapon to be better than the enemy's weapon can give you a better weapon.

3. Sometimes, it wasn't possible to produce a new weapon in sufficient numbers to go close to replacing the weapon it was intended to replace. Converting factories to produce a new weapon takes time, and that time isn't always available. Thus, an old weapon can continue in production and use, even if there is limited production of its replacement. This is the case both where the new weapon is a direct copy of an enemy weapon, and where the new weapon is a new design.

Each country had their own problems, supply issues, and other factors that limited their armed forces. Sometimes just copying one piece of tech is simple enough, but to copy an entire design and try to fit into your doctrine and industry is a whole another issue.

The Germans seeing the British tanks for the first time was a shock for them, but they had to deal with it then and there. While the British infantry tanks were better armored, they lacked the organization and tactics the German Panzerwaffe had. The German Panzerwaffe used a combined arms doctrine with rapidly deployed infantry, artillery, and CAS to support the tanks. British doctrine was a mix of infantry tanks which were slower but heavier supporting tanks with infantry, while using cavalry tanks that are faster but less armored to breakthrough gaps in the line. But the British didn’t embrace combined arms warfare early on, whether it was a lack of troops to support the operations or a decision in the High Command. This became a glaring weakness that Germans exploited early on. Now the Germans did want a breakthrough tank and had the Tiger I in development at the time, but for now they couldn’t really have a design thrown into the Battle of France as it was in full swing and later ended in a quick victory.

Speaking on the German T-34, there was an actual attempt to make one after the T-34 debut. Debate was to make a T-34 like tank or the Panther tank. The arguments boiled down to quantity over quality of tanks, with the US and USSR bringing their industrial might down on Germany a decision needed to be made fast. Germany had no illusion it could keep tank production with both powers and a T-34 like tank with more coming off the lines but in lesser quality looked like a lesser option. Germany was running short on war materials such as high quality alloys and quality iron for steel, not to mention a lack of oil by 42-43. This meant even if the Germans a mass produced tank they would lack in quality and would not be able to field large numbers of them due to oil shortages. The Panther became the number one choice after this, however the T-34 threat would influence it’s new designs. The 29 ton Panther was slated with more armor, which added an extra 10 tons to the tank. This was not factored into the new design and the Panther’s parts which were designed for 29 tons, now had to add 10 more tons which stressed the design. The Germans also added armor hardening techniques, which helped Germans deal lower quality metals with a better chance of deflection. But it came at the cost of the armor becoming brittle in some places and were more susceptible to HE rounds and would cause large holes that couldn’t be repaired easily. While T-34 tank for the Germans sounds like a good idea for the outnumbered Germans, they went with the Panther which had a lot of issues but it’s range and armor help keep it competitive in the defensive war the Germans now faced.

In the end what it comes down to is what the country needs during the war, rather than what looks cool right now. For example the US used the Sherman for most of the war because it was reliable, easy to maintain, very survivable, and could be mass produced quite easily. With the US needing to send a tanks over 2000 miles away, they needed to work first time, every time. The Germans used the Panther because of it’s armor and gun that could outrange any Allied tank and usually stop any rounds at it’s front. While it’s range and reliability were horrendous on top of a lack of spare parts at times, this was mitigated by the German rail network bringing the tanks as close to the front as possible. Being in a defensive war as well helped as the German panzers could set up ambushes and defensive positions to lure in Allied advances. Switch the two tanks around for both nations and neither would work for their respective situation. The US had the Pershing tank which suffered many reliability issues and was very heavy, this was their Panther but it didn’t make it to the front in large numbers until the end of the war. The Germans wouldn’t benefit from the Sherman due to their increasing shortage of fuel and their industry being bombed. After the war the Allies would look at other designs and see what they did right and wrong, but during a war it is best to go with what you know and what will work for you.