“Big” movies are big business. They have to pull in the widest possible audience in order to make a return on the huge investments that are made in them. The ratings system is in place to give some kind of ‘known’ guidance to the viewing public about what they might expect to find in the film, and it’s weighted to appeal to the way most American families raise their kids – meaning that “depictions of violence are okay” and “depictions of overt sexuality and nudity” are not so okay.
As long as Americans keep supporting these kinds of “big” movies, they’ll be made the way they’ve always been made, because they sell. If you prefer foreign, independent, unrated, or more nuanced movies that are more character and plot driven, whether or not they have sex and nudity and whether or not there are graphically violent scenes in them, then you have to seek them out instead. But most of us aren’t willing to make the effort to search for or screen movies that way, or to “seek out” something that takes longer to find and is more difficult to schedule than what’s showing at the multiplex at the mall.
And most Americans still won’t let their kids see nudity, whether or not there’s “sex” involved or not. Even those who permit its depiction in their entertainment are generally unwilling to overtly acknowledge it.
So, yeah, it’s “Puritanism” at base, but it’s also economics, politics, marketing and convenience. Like everything else, it’s not a simple “here is the reason” answer – no matter how much people prefer that to actually thinking about a thing, too.