This is something that always bothered me while watching movies about Americans vs Red Coats, or French vs British etc, where you'd see lines of men facing each other in battle. Now the tactic obviously is a very effective one when it comes to facing a disorganised army, but when facing an army using the exact same tactics, it seems stupid and even a crazy way to fight. I can imagine being in those front lines asking myself "why the f**k are we marching forward like this without protection again?".
Seems to me that it would be very easy to invent something like a mobile shield wall. Basically a metal plate on wheels, perhaps with holes in it so you could fire at the enemy through them. And they'd have no way (other than canons) to get at you. Especially if you were a disorganised army like perhaps the early Scots, that sounds like something any sane person would come up with as a countermeasure.
So why wasn't it? It seems like it was either digging in and shooting at each other, or that crazy marching at each other tactic :-)
I would suggest you look at this question where someone asked the same thing while also looking at this answer and this one as to the methods and purposes of warfare during the early modern era.