Cultural standards, man. There's a lot of premium placed on women's sexual propriety and there is also a lot of importance on not disgracing your family in general. Like, in the US, your parents might be mad that you showing your puss on OF, but it's likely that eventually they'll get over and it and be like "I don't agree with what she is doing but she is still my daughter and I love her and I support anything that makes her happy, as long as she is staying safe, blah blah blah."
Africans don't play like that. If you are whoring online for money, your parents, your whole family, maybe most of your friends and associates will all cut you off, probably forever. And the culture is so family-oriented that it hardly is worth it to be raking in all that simp cash when nobody in your family or community fucks with you anymore. Also, they probably wanna get married some day and there is the idea that no man is gonna marry you after you done already spread your ass cheeks on the internet. (I've noticed that some of the chicks who do go all out on OF are the kind that are already married to an older white simp who probably gets off on it)
South Africa is different in some ways... in a lot of ways their culture is a bit more liberal, a western-oriented (though there is a limit even to that). A lot of SA chicks hide their faces because there IS still a stigma on being a whore. plus, there is so much sexual violence against women down there that they're risking getting harassed (or worse) by these aggressive-ass SA niggas if they're recognized as presenting themselves as being "open for business" in that way.