There is a distinction between a blunder and a mouse slip though. The TOUCH MOVE rule in chess is touchinga piece with the INTENTION to move it, Accidentally touching a piece does not count. The equivalent online is a mouse slip. While mouse slips are expected in 1 min games in 5 min or longer games the ability to at least offer a takeback to a player is a sign of respect to your opponent. (In my experience mouse slips are fairly obvious, stopping 1 move short of a capture for instance) My guess is that the programming is a bit more intensive since it would require time stamping each move something that this site doesnt seem to have.
A way to take back a move

All I have to contribute is that on chess servers with takebacks many people ask for a takeback and then don't even move the same piece...And they also get rude when you don't give them a takeback. I happen to like chess.com's no takeback rule :)

Actually, Chess.com does allow takebacks. You have to set up the game that way before starting it, and it's only available in unrated online games.

Its not even really a exclusive chess.com rule. Its a rule of chess, you touch a piece you can only move that piece. Once a move is made its set in stone that's why one should think carefully before making moves. I had a friend who erroneously thought takebacks make you a better player because you get more interesting games. What it really came down to was he just didn't want to record the games and iron out his mistakes. Very in the moment, which isn't a bad thing but natural talent can only take you so far and if theres no impetus to correct mistakes, no one feels a need to improve.
I wanted to post this here, since a lot of people that I've played with are confused when I explain this. The situation is the following:
You just made a terrible move in a friendly game (even if it's rated, who cares about ratings). But you didn't capture anything, and you moved a piece other than a pawn. We can undo your blunder if my next move is an insignificant one (place a knight or similar somewhere), after which you get a chance to move your piece back, which I will follow by moving my piece back. This way we land in the exact configuration before you made the blunder and all is good.
Have a wonderful week :)
gxh8Nmate