Why would they do porn? If you're a good looking guy, you can have sex with good looking women and not torpedo your future career options.
It makes sense a young fit black guy in a Japan would not work in JAV even if he was offered twice the rates of a famous Japanese male pornstar. Do you want to become an accountant or teacher or something else and get paid well while having sex with lots of women or do you want to always be worried about your next 'job', live in a 1 bedroom apartment and have worse sex with women that might look a little better thanks to a professional makeup team?
If Japanese men aren't falling over themselves to get into porn why would someone with fewer opportunities jump at an offer from JAV?