Because weebness, and because roleplaying with the visage of IRL people will always be weird lol. Doesn't matter how famous they are. I've only used IRL fc's in one site and I did it in the name of diversity because there were, as usual, white faces everywhere. The one white guy I use, who was mostly a meme because he was an internet personality, turned out to be a big AH so I took that down fast. So I had Tessa Thompson, Pedro Pascal (who I really only ended up using bc the Filo actor I wanted just didn't have pics I can use, so he was the closest), a black actor I forgot bc I just found him to replace Chadwick Boseman when he passed, and Chella Man, a trans actor, and another trans model who, for some reason, isn't in my pinterest anymore so I forgot their name. Oh and I had a K-pop face lmao because I needed someone with white hair and I was only really comfortable using Asian faces (black isn't asian but racially closer to us here although i'm actually one of the paler filos around but that's nothing unusual here)... though either way, I am down to just using aesthetics for them.

Weebing aside though, art just has more expression. There's no way IRL pics can compare to it. Animanga also tend to be less serious, and when doing the reverse, like animanga imitating life or IRL imitating animanga, the former feels perfect. IRL feels more stuck in its scope, which, you know, is part of it. It can't compare to animation.