What I mean exactly is why did armies go about as large masses to fight set piece battles, rather than smaller groups over a wide area?
Because:
If an army is spread out as smaller groups over a wide area, a more concentrated force would always win an engagement if they were to meet. Armies are difficult to raise, expensive to maintain, and attrit at an alarming rate. Sometimes up to 30-40% attrition rates per year mostly from desertion during the early-modern period. If you're raising one you're using it now and decisively at that.
They did on top of this all. With the brief exception of 1618-1648 (and arguably even then) the period of ~1500 to 1792 was distinctly indecisive, slogged down siege and raid warfare. Arguably 1300's and 1400's but that's outside my wheelhouse. Armies would slowly but surely siege each others fortresses and send hundreds of smaller raiding parties into enemy territory in a kind of persistent warfare. Even in sieges the occupying army would fight with locals and enemy remnants while foraging the area for food etc.
If we look at WWI, arguably the first war with the kind of 'front' that you describe, what has truly changed? What about WWII? There are divisions, corps, armies, and army groups. These armies have relative autonomy and perform their own campaigns and fight battles. The difference is that in 1914 there were 884 000 French troops ready to go split into numerous armies that permitted a consistent front. In 1700 there were only a couple of armies, of significantly smaller size, with a few ten thousand men operating at a time at best, that a State was capable or willing to raise and thus that permitted much more economic use of those forces along with smaller frontages. It was so rare to get an army like this together 'back then' that if you were going to fight it was going to be under ideal conditions in a 'set piece' battle.
A lot of people struggle with this concept I find. It's not like in 1914 things just mystically changed and the entire premise of army organization shifted. What was shifting was the amount of armies available to a government and how quickly and cheaply they could be created and organized.
H.W. Koch, The Rise of Modern Warfare: 1618-1815
Christopher Duffy, Fire and Stone: The Science of Fortress Warfare 1660-1860
I read that in medieval times the majority of battles were sieges anyway so there was no chance of "missing" the enemy. You knew were they would be ahead of time.
Also marching large armies on foot would always take a great deal of time and scouts on horsebacks could quickly inform their own generals where the enemy army was heading if he prefered to meet them in field.