I play a lot of boardgames, and there is a lot of rules involved. Usually when getting a new game I like to read the rules first thing after unboxing the game itself regardless whether I'm getting a chance to play it anytime soon. Even when playing somebody elses games I like to read the rules.
But there are a few popular games I have been playing for years. So e that I have never actually read the whole rulesbook save the few passages to check some particular rules. Some games have rules passed as "heritage" knowledge. There might be a long chain of people who have just been told the rules by people who themself haven't originally read the rules.
Especially when playing with multiple groups this can lead to interesting rules conflict where each player has learnt the rules a bit differently.