It's 2021, and Instagram Still Won't Free the Nipple

Last week I was having a conversation with a female friend of mine. She's a hobbyist model and will sometimes pose nude. She also likes to share her images online. Going through her portfolio, I wouldn't describe any of her images as sexually explicit. But yet, when sharing them online, specifically on Instagram, she still has to sensor certain parts of her body. Why is this still happening?