There have been other discussions on this. Basically, there seems to be many reasons.
Personal preference (is that racism?)
Pimp in the background (assuming black, so that is likely racist)
Avoid being harassed by pimps and thugs (some ads do say no pimps or thugs, but some fake ads say 1000% real pics, so who knows)
My favourite thought, which has never come up, is that to a scammer/faker, likely seems far too risky to rip off/piss off a young black dude who doesn't put up with that kind of shit (is that racism or reality? lol)