why do grandmasters usually have higher blitz ratings than rapid

It’s because rapid rating after 2300 or so becomes extremely time-consuming to increase rating due to how long it takes to get an opponent; the higher rated you get, then the longer you wait.

It’s amazing that chess.com has millions of members, so you never have to wait long for the computer servers to pair you against a similarly rated opponent… but imagine you’re Magnus Carlsen. You don’t have many similarly rated opponents in the world. For a close matchup, you’ll have to wait for Hikaru Nakamura to be online, or some other elite Grandmaster. They have to be online and seeking the exact time control you are at that exact moment.

That’s why you’ll see high rated titled players typically either:

• Playing other titled player friends because they agreed to play first to 10 wins, or something like that; that way they don’t have to wait for new opponents

• Playing tournaments like Titled Tuesday (which is now on Thursday and is also not rapid, but the point is events they know they’ll have high rated opponents to win rating from)

• or switch to popular time controls… which at the higher levels tends to be 3+0 blitz or bullet chess.

That’s the main reason you’ll see high rated players switch their primary time control to blitz or bullet. It’s not that they “can’t” reach higher rapid ratings; it’s just that they typically feel the grind isn’t worth their time. They’ll often play speed chess online for training, or for fun and they’ll play in-person OTB events for longer time controls.