making an engine for variants is ridiculously hard and more complicated than making a strong regular chess engine. the engines made for 4pc teams are user-programmed and are not affiliated in any way with Chess.com or their staff; they are simply hobby projects made to be tested out against human players
Why there are not more variants AI

Yes, variants have more rules which could make them more complicated. Another reason is probably the fact that there are waaaay more people playing normal chess than variants. It doesn't make sense to devote resources to programming tough variant AI's when there aren't nearly as many people playing variants. 4pc bots would probably be even more complex because the goal isn't just to win, but to get points.
Why don't Chess.com make an AI for less popular variants????
What do you think???