Same reason anything else is. It’s big money. Porn is a multi billion dollar industry.
As for why “extreme” porn is so “normalized” I think it’s more because it’s stigmatized ironically. Have you ever had an in person conversation about why there’s so much step family porn? No of course not because then both people would be acknowledging they watch porn for one. So the social factors that would guide things are totally absent.
Large Hadron Collider could be an interesting candidate. It’s not very visible, but it’s unique, gigantic, and insulated from damage