San Andreas was not silly? Wut? If anything San-Andreas took it to new levels.
Sneaking onto a aircraft carrier to steal a harrier? Chasing down Ryder on a boat as he yells "im a motherfucking genious!"? Having girlfriends that are each into different types of dates (including one that PREFERS to go to a fast-food joint versus a nice place)? Attacking Area-51 to steal a jetpack? Stealing a crop harvester and proceeding to run over the farmers who try to attack you? Having rough sex with the antagonist of GTA3 before she falls for the GTA3 dude and you have to streetrace him before they both run off to Liberty City (preceding the events of GTA3)? Working for a blind Asian billionaire who refuses to admit that he is blind while everyone pretends not to know to make him happy? Stealing the rhymebook of a famous rapper (in a splinter-cell-like mission) for your looser friend only to later save the rapper's life as he tries to kill himself when his career falters, only to steal the rhymebook back from your looser friend to bring the rapper back to popularity? The missions were completely nutters! What game did you play?
Oh no, you're absolutely right, but alongside that was a more serious "plot" and overarching main story than in Vice City, which was entirely tongue-in-cheek. All of these were on the one hand, where it's a crazy, silly world, and on the other hand was a serious attempt at storytelling, and it felt completely bipolar.