[amalcon]

I think there's a third, bigger reason: FIDE rating is based on classical time controls. Nobody uses classical time controls online for various reasons. Even rapid is a somewhat different skill set than classical.

Of course these things are correlated, but e.g. there have been times in the world championship when one of the players thought about a position for 20+ minutes before making a move. That's hard to do, and requires very strong visualization skills.

[tehnub]

There are FIDE ratings for other time controls too. Here are the top blitz players for example: https://ratings.fide.com/top.phtml?list=men_blitz

[amalcon]

Oh interesting, I didn't know that. Thanks!