When I read about ancient battles on wikipedia, the losing side's losses are almost never their full force. Usually a good chunk of the army escapes, many of them infantry. How did they do this?
Once the losing side started running away, I would assume this would give even more of an advantage to the winning side. It's MUCH easier to attack someone running away than it is to defend against someone running after you.
I can understand the cavalry getting away but many of the survived were infantry. Did the winning side not chase after them? If they did, what prevented them from mowing everyone down?
So in other words, when an army lost, why wasn't EVERYONE wiped out on the losing side?
You are going to have to give an example of this. From what I have read this is the exception not the rule. Especially 'ancient' battles in the Hellenistic era involving spear formations. (where vast majority of the losses are taken AFTER a formation breaks/routs)
Well I know that it was common for the Romans at least to leave a sizable force to guard the main camp when a battle took place. This could be a new and unexperienced Legion or Cohort. I would imagine that when they saw the battle was going to be lost it would be easier for them to escape seeing as they're further away from the battlefield.
Can't speak for the battlefield portion though.
Ignoring cavalry - the guy who's dropped his weapon and shield and is running for his life is lighter loaded than the guy with his weapon and shield who's chasing him.
Plus, if the pursuers completely abandon their unit cohesion to chase as individuals, they risk running into a formed enemy unit and getting chomped.
Cavalry, especially light cavalry, changes this equation, and many historic massacres featured infantry formations getting routed then hunted down relentlessly by light cavalry formations.