I noticed that if you play against the higher computers like stockfish, if you start the game with a weird move like h4-h5, they spend time. So, it would be easy to beat them in 10 sec. That's just a theory though
Why does a computer decline your challenge if the time control is under 1 min.?

Title says all
Probably related to the fact that latency to opponents are variable and there isn't a good way to configure them to not time out all the time.

OK, thanks, that makes sense now. Why are the computer's rapid ratings all so low though?
The engines in Live get their ratings just like members, by winning and losing games.

and could get flagged or farmed lol
This game was unrated but still I won: https://www.chess.com/live/game/5647590678
maybe chess.com wants is to play slower time controls? (I'm not sure)