Probably because business efforts behind these domains are rather transparently profit-seeking and wired for the short-term, make a quick buck, approach.
Also there's a good amount of technological quackery that goes on in the hyping of these domains to not-so-tech-savvy investors who don't realize that a typical 14yo kid understands intuitively that "sex.com" is probably a crap parked site or link directory, whereas pornhub.com, on the other hand, provides a service that 14yo will love.
"..and we've got a GREAT domain name, pets.com, I mean, if you want a pet, where you gonna go? We're right there."
Labors of love are what win and enable you to eventually fleece your customers and have them thank you for doing it -- not having a 13 million dollar domain.