Pistols often were carried in multiples, sometimes with an aide to reload them while their owner was firing them. However, these were relatively small, portable weapons typically used by wealthy owners.

The big issue of having infantry carry multiple weapons is cost and weight. A flintlock musket is a bulky, heavy thing to carry around with you - a typical Brown Bess musket of the period weighed about 5kg and was around 1.5m long (or 10lb and 5ft respectively). Carrying around two muskets is more likely to hinder than help a soldier in combat, especially as he will still need to go through the process of reloading both of them after he's fired two shots. There's also a logistical issue - equipping your infantry regiment with two weapons each makes it almost twice as expensive as just giving them one.

As for developing a weapon that could fire without reloading - this was borderline impossible in the 1700s. This isn't to say that gunsmiths didn't try, and there are a few examples (I've seen one from as early as the English Civil War) but these were very expensive, complex pieces with dubious reliability that could never be deployed en mass on a battlefield. The big difficulty is black powder, which produces a large amount of particulates with each shot (most obviously as smoke). This will quickly foul a gun barrel if it's not cleaned between each shot, to say nothing of its impact on any attempt at a complex automatic mechanism, which effectively prevents the creation of any sort of repeating firearm. Another way to get a multi-shot firearm is to simply add extra barrels (the most famous example being the seven-barreled Nock volley gun) but aside from the issue of additional weight, firing one barrel at a time would be quite a complex thing to achieve since a separate flintlock would be needed for each barrel. In practice, guns like the Nock had one flintlock and fired all seven barrels simultaneously - which could have a devastating impact, but also produced infamously high recoil.

The Gatling gun itself was not invented until 1861, enabled by considerable advances in machining and the invention of smokeless powder.

So to answer your question - the methods they did use were the best available at the time. Standing in lines may seem suicidal to modern sensibilities, built as they are on ~150 years of accurate rifles. However, muskets of that periods were far less dangerous, and there are considerable advantages to fighting in regiments (in particular, it's a very good way of manoeuvring conscript troops efficiently and ensuring they stay in line when fired upon, as well as warding off cavalry charges). /u/dandan_noodles goes into more detail here.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!