Rebuild makes the game much shorter if you do it deliberately. I doubt the bots were only trying to trash the Province pile, that wouldn't necessarily be a good idea anyway if they have each other to compete with. Also, in a three player game, there are 12 Provinces instead of 8, so it takes longer to trash them all (unless both Rebuild players cooperate, and it's a prisoner's dilemma). Ambassador also tends to make the game much longer, which is great for a Gardens player, besides the obvious reason of increasing his deck size.
Ambassador and Rebuild are each very good cards, but they really antisynergise - Ambassador wants to give away all your Estates, but Rebuild wants to turn them into Duchies. Also, Ambassador wars can take until turn 10+ just to hit $5, whereas Rebuild wants to hit $5 as early and often as possible.