There's a good reason for doing these types of engagements. First things first: The rifles were inaccurate, people had to be close enough to clearly see each other to hit a person. It takes a person out of battle 20s to reload a rifle but raw recruits with shaking hands take a lot longer.

So here's what you would like to do with an army. Shoot them all down before they reload or even engage them in melee with bayonets to create a breach and crossfire the side of the opponents line.

To make this more effective you stack as many guns as you can in as little space as possible and use the men as cover for the men behind.

There is of course space for guerilla warfare but if you want to take out a big army of flintlock muskets you cram everyone into a line and blast them. The other side does the same since it's the current war meta and you end up with 2 lines of people lining up and shooting each other.

So why not just guerilla warfare? This comes down to the same reason why castles forts and fortified cities were important in medieval times.

To win a war you take the capital, large cities and whatnot. To take a major objective you need an army. There's no army without supply. Running supply lines between forts means you won't really get a lot of them. So you need to prevent a big blob from taking them. You can't win with guerilla warfare fast enough before the opponent takes major objectives.

That's how you end up with the meta from 1700 until bullets.