Rapid 15|10 is the best online surrogate for 90|30 over the board classical.
Blitz and bullet require different skills: moving fast, flagging, premoving,
which have nothing to do with chess as such.
why do people play rapid

I play rapid because I am slow in thinking. I am already old and I am not fast at moving my mouse. It also makes me relax. I am always panicking during time trouble and it is not good for my heart.
For me personally, improving at chess is fun and I feel that that is more likely to happen by playing more rapid games than blitz/lightning games. The fact that I'm unlikely to ever hit master level is irrelevant.

Rapid gives you time to think. Online bullet is more like a video game then it is chess. It is all about who can move the fastest not who's the best player.

I highly doubt most people will be at the top level so why don’t we all just have fun and play blitz and bulluet
Strange question... It's like saying: I highly doubt most people will be at the top level as tennis players so why don’t we all just have fun and play ping pong.
There are those who have fun playing ping pong, those who enjoy playing tennis, those who enjoy playing both...
I know rapid is for chess improvement but I highly doubt most people will be at the top level so why don’t we all just have fun and play blitz and bulluet