Botvinnik said that bliz is bad for one's improvement.
Elite GMs can play strategically, because they have tons of OTB experience and they have extra level chess knowledge. We, mortals (who are under 2700 FIDE elo), don't know that much. And a lot of positions which are clear for elite GMs are unclear for us. This is a point.
Personally I play blitz games in order to improve my opening preparation. But for general improvement I play rapid games with my friends (it's hard to play rapid with random people for many reasons) and OTB games. And the thing which helps me the most is studying chess in all aspects.
GMs don't play rapid because at the higher level it's so easy to cheat effectively. Players under 1600-1800 usually don't even know how to cheat in the game. Or their cheating is super obvious so they can be caught easily. Apart from some serious tournaments, it's widely considered to be a beginner's time format. Most advanced online players nowadays prefer Blitz, that is self-evident. I don't believe cheating has anything to do with it.
For instance I can simply know that my sacrifice on h7 is going to be successful not because I've calculated 5 different variations in 5 seconds, but because I have the experience in these types of positions. And that is exactly how you learn. What you meant by "having experience in those positions" was memorizing them. That is what happens when you play under time pressure, you are forced to prepare your lines beforehand and ultimately internalize them faster, because you have to memorize them first. That is imo the fastest and most effective way to learn chess. Not sitting in front of a 30min game when you simply don't know anything yet. Play a lot of fast games and study as much as you can, that's my way. I just made Fide 2227 in Blitz so I absolutely believe in this method.