If I join (say) a book club, I think it's reasonable to expect that I will read the books. It's not acceptable just to turn up to book club and not be unpleasant.
I don't think this is expected in most book clubs and if it is it should be stated.
If I join a choir, I have to be prepared to sing, to do whatever warm-ups we do in choir, probably to do some exercises and practice in my own time.
I don't think yopu are expected to practice on your own time (or at least I wasn't in 4th grade), you are ex-ected to sing but not to be on key.
Obviously every RPG group has its own standards, but I don't think there's any general assumption that no one is expected to take it seriously, or that it atypically demanding to expect more from a player than not being unpleasant to others.
I think there is no general assumption that players are supposed to take the game seriously. What I am saying if that is an expectation, that you are a group of "serious" players that needs to be stated up front.
This sort of attitude that there is a minimal acceptable standard is a form of gatekeeping and one that keeps newbies interested in the game away. It was prevalent in the 90s and 00s but is less so now I believe and I think that is one reason in the explosion of popularity.