It’s street slang, for the same reason Cockney rhyming slang exists: solidarity and so that their so-called “betters” don’t understand them.

I imagine kids/teenagers originally, as with many linguistic innovations. But of course such people grow up and their conventions become mainstream.

What do you mean "who has time"? It doesn't take more time to e.g. say "meuf" than to say "femme".

spanish has the same thing where you call your friend "chupa nepe" instead of "chupa pene".