Because horses don’t let just anyone grab them, many run off, and others are taken by troops who had their horses slain. The issue with any of these “realistically” assertions is they ignore many other “realistically” counter angles.
For instance: every few weeks you’ll see someone complaining about the amount of recruits that get refueled into towns and villages and how “unrealistic” it is. Yet, they almost all ignore the amount of released prisoners that get discarded, the amount of bodies that get sold to the brokers, the amount of prisoners that never get taken with you (just because you imagine you killed them all doesn’t mean that’s what the droves of merciful nobles also did) etc, etc, etc. And that’s all before you factor in how rare annihilation of an opposing army is “realistically”. There is give and take here.
So since your troops don’t need you to replenish their horses at all once they are upgrade to their current point, it’s a trade off that you don’t get 50% of the horses that the opposing army was using, or whatever percentage someone thinks they ought to get