There's still a sizable portion of the feminist community which is sexually puritan and want to ban porn. They are quickly becoming antiquated and outnumbered with the new generation, but they are there. There was some groups opposed to Fifty Shades of Grey not because it was terrible, but because they think all BDSM is rape or something.
Remember also that most Hollywood producers are also old and old-fashioned. There's so much clean prospective talent out there there's no reason to go rooting around in the trash, unless you're doing it on purpose like Paris Hilton in Repo: A Genetic Opera.
There's also the issue of really how big and viable the porn industry is now. It's certainly huge, but it's split in a million directions these days. There's no such thing as mainstream porn studios or "porn stars" as much anymore. Sasha Grey was born at just the right time for it. But now everything seems to be focused on amateurs and cam girls, and there's only so much money in that. Certainly nothing compared to Hollywood bucks.
But, it could still happen, for the reason >>78351 state. I would say it's more likely to happen with the pseudo-acting "famous for being famous" community, like the Kardashians or something similar. I wouldn't hold out hope a J-Law or someone who has been to the Oscars doing it, though.
Remember McDowell didn't exactly have an illustrious career. A Clockwork Orange was also super sexful, which is why he could only get small B-movie parts. He did have a comeback, but that was only due to his voice, which got awesome with age.
Porn is also defined as intrinsically not artistic, but Caligula offers and interesting case study in that. Is a movie not porn because there's historical costumes and sets? There's not much entertainment value in Caligula outside of the shocking sexual and violent antics. I'd say the first part of the movie is serviceable but after that it just becomes all porn.