Why are my AI enemies surrendering all the time?

Sounds like a bug to me. In fact, it sounds quite similar to what's described here (scroll down a bit to the "AI Update" section):

Age of Kings: Downloads

The Age of Empires II AI Update - This update addresses the issue where the computer player resigns after a few minutes of play.

That page has patch 2.0a, which applies to Age of Kings. You can also find that patch here:

Age of Empires II Update v2.0a

If you have The Conquerors expansion, you'll want to go with patch 1.0c instead:

Age of Empires II: The Conquerors Update