Why? Well, despite all of the speeches by Mr. Braveheart about the glories of combat, in reality, you always prefer the path to victory that doesn't get half your men hacked to pieces -- armies, after all, are expensive. So rather than agree to have your 20,000 men meet their 20,000 in a scenic open field, armies would only look to engage if victory was all but assured, in which case the weaker side would try to hole up in a castle and turn the affair into a (usually boring) siege. Otherwise, open combat was to be avoided if at all possible, with leaders much preferring to pick away at the enemy with raids, weakening them one razed village or captured stronghold at a time.
Yeah, most of the fighting wasn't exactly the kind of shit they write heroic poems about. Pillaging was the primary tactic of the day, mainly because while we like to imagine epic conflicts fought over freedom, faith, and murdered princesses, most wars were fought over economic issues like property rights. Since peasants were essentially a resource (they provided food and tax revenue), terrorizing them was an easy way to hurt the enemy without the risk of taking an arrow to ye olde gonads.
On the rare occasion when large battles did happen, only armies commanded by total bozos would descend into the usual trope of two forces that fire a few rounds of arrows and then start hacking at each other in a chaotic free-for-all. Scouts and spies were used to track enemy movement, armies used terrain, trenches, and tactical positioning to try to gain an advantage, and they would, you know, try to hide their forces if possible. Commanders studied past battles and textbooks were analyzed and revised. Again, they couldn't throw waves of men at each other. They didn't have them to spare.
So every scene in which a hero leads a courageous charge into the teeth of of the enemy is actually showing that he's kind of an idiot. A true hero would be slaughtering confused peasants and stealing their cattle, dammit!