Where I live even full nudity would not be considered obscene at all, unless something sexual was going on.
However it is becoming more and more the case. With American platforms and media, their learned attitude of being overly prude also made their way here. Though I'm not sure whether "prude" is the right word to describe the crippling self-consciousness many experience when faced with nude humanity.
I suspect it has be actively instilled by your parents and society around you while you are still young, or you won't ever have it.
"It's not the nudity that's bad, it's the sexual provocation! I mean, I don't care if they're topless in their own homes, but do they have to flaunt it? They're so in-your-face."
It's not difficult, it's the exact same kind of decision they decline to make with regard to advertising. These days that refusal lives mostly in political and health messaging, but we can see that they don't refuse in a lot of other subjects.
It's more than that though. It's not just the quantity of stuff you wouldn't want to see, but that FB would then be known for that type of content, and they'd lose the other content that brings in the money. Purely from a business model perspective, they want to keep that stuff off.
I mean, that's pretty arbitrary too, e.g. FB censors female breasts, which is not considered obscene in many places.
It's a difficult line to walk.