AI cavalry does not know to disengage from an unwinnable fight against spearmen.
Some of the older TW titles had this issue, some do not. Those that do are stupidly easy to win.
ThroB has this issue.
All I need to do is pair my cavalry wings with any old cheap spearman unit and they will always win the fight against the enemy cavalry because the enemy cavalry isn't smart enough to break off the fight, leaving my cavalry free to roam the enemy back line.
It's so routine:
1. Friendly cavalry engages opposing enemy cavalry
2. Friendly spearmen catchup to the fight and its over in seconds
3. Friendly cavalry win battle 30 seconds later
Because of how fast this happens, I can be grossly outmatched in the center infantry fight and still win the battle because my center only needs to hold for about 1 minute before the cavalry comes to the rescue.
If the enemy cavalry were actually smart enough to disengage from the spearmen, they would delay my own cavalry's ability to roam.
Why isn't the BAI keeping anything in reserve? If they outnumber my army 3 to 1, you'd figure they'd leave a few spearmen in reserve so that my cavalry can't run amok.