The somewhat lame answer is that they didn't fix them because doctrine said not to (Except the Soviets, which I'll address). As for why doctrine was against this, there are many reasons why you would not want a bayonet attached to your rifle. For starters, it adds to the length and weight which can make it unwieldy. If you've ever handled a rifle with, and then without a bayonet, the difference is very noticeable. The instances where a bayonet actually would prove to be useful are quite few actually, so those few chances where you have an advantage are easily canceled out by the at best annoyance of having it on, and at worst, numerous accidents where you end up poking your buddies.

There is a bigger matter to address though, and this is the concept of barrel harmonics. For a really good explanation, you'd need to go to /r/guns or /r/askscience. While I can explain the basics here, that is where you can get the nitty-gritty. So basic idea is that when you shoot a gun, the barrel vibrates. There are ways to limit the vibration such as using a heavier barrel, but you really can't totally eliminate it. The vibrations do affect the accuracy of the gun. But, since the vibration of the barrel should be similar every time you shoot, so when you zero your sights, this vibration isn't going to affect your point of impact too much. However, when you stick a long piece of metal onto a rifle, this changes the vibration, and it will change the point of impact, so you would have to rezero your sights, or else use some Kentucky windage, in order to use the sights correctly. So basically, to ensure consistent and accurate fire, you need to be shooting either with the bayonet always on, or always off. Or else you need to adjust your sights every time you switch it on/off, which is not easily done in the field, under fire! In the case of almost all military firearms, they are zeroed to be shot without the bayonet attached. Now, of course if doctrine was to have the bayonet on at all times, the rifle would be zeroed to fire with it on, but as noted before, having a bayonet on your rifle is a hassle!

But, as I said, there is an exception. Russian doctrine since at least the early 19th century - and which was inherited by the Soviets - was to have the bayonet attached at all times. This goes to their belief that the bayonet was the true strength of the Russian soldier - Suvorov has a famous quote that is always awkward in translation, along the lines of "The bullet is a mad thing; only the bayonet knows what it is about." The Russian would say this is because Russian soldiers were the bravest and willing to close and use steel, while the cynic would say this was because Russian marksmanship was appalling and it was the only way they could kill anyone. The origins being before the period I concentrate on though, it isn't a debate I'll wade into. What is important here is that it stayed around into the Second World War. Initially, the M1891 was not even issued with a bayonet scabbard, since it was never supposed to be removed except for maintenance/cleaning. As such, the Russian and then Soviet made Mosin rifles were zeroed in to shoot with the bayonet attached - here are some Mongolian soldiers in 1939 demonstrating this.

So to TLDR this, the answer is three-fold.

A) Rifles were generally zeroed at the factory to fire with the bayonet off.

B) Although in theory they could be zeroed to fire with it on, doctrine was against this, since you underestimate how annoying it is to have bayonet attached.

C) The Soviets are the exception, since they remained attached to a doctrine that may or may not have stemmed from the need to get 19th century Russian peasant conscripts super drunk and make them charge the enemy because they couldn't hit the broadside of a barn.

Your central question has been answered excellently, but perhaps another perspective on the cultural history could come from Joanna Burke's "An Intimate History of Killing" where she points out that bayonets had a big historical/psychological presence (newspapers love to write about them), and bayonets were popular as an idea in the upper ranks ("inspires fighting spirit!" they thought), but practically a bayonet is not very useful with modern firearms. Basically she points to bayonets having a big propaganda value: British Home Guard literature was super into knife training/bayonets, to the extent that they annoyed the regular army who was actually involved in fighting.

Michael Stephenson's "The Last Full Measure" has breakdowns of fatalities in different wars, and I can't recall specifically which percentage point is tied to bayonets in the WWII, but it was negligible. Bayonets and bayonet charges made great stories, but they were super rare for Allied troops.