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

The AI could have triggers built into that after x amount of time...if they are too far behind in score. It will surrender because the amount of time it would take the ai at difficulty level to catch up to you will take too long.

There are a few reasons why the AI surrenders. Some ways that help you avoid this issue include: do not place computer players starting town too close to the edge of the map. Also, the computer will give up when its bank of resources is depleted. That is the short answer with out all the math. Try to give a starting bank of 200 food and wood. Lastly, if you have done these things and the computer still gives up, check for wolves. sometimes the computer will do some crazy pathing, It may be running all of its starting villagers past wolves.