Without understanding the basics of the map editor it can be tricky to understand why the AI's tendencies. Nevertheless, I shall try to explain.
The AI will recruit horses if the conditions are met:
#1: The AI has the resources for either a scout or knight
#2: The AI has not reached the set maximum trainable units.
#3: The AI has a place to place knights or scouts. That is, the AI has mounted unit defensive positions or is set to "auto defend"
My guess is that you are having trouble with #3. If you are playing a regular multiplayer map (one normally played with humans) then they AI should be set to auto defend and won't need defensive positions. However, many maps have defensive positions. If yours does, check to make sure there are some mounted defensive positions because the AI won't train units if it has no place to put them.
This is my best guess. I could be way wrong though