why the hell don’t they just ban the accounts that try to send nudes to minors?
Because they are sent from minors to minors too? Teens are horny, they copy adults making nudes, sometimes just sharing porn. Recently there were problems with classmates using pornLLM to undress their peers. The abuse problem is harsher, but I feel it's the minority of nudes received by minors. Honestly, I'd have changed the EULA to forbid it on a public service like Insta, because unlike messengers there is everything to be deanonymized and explicitly targeted by an abuser, including stalking and threats IRL. For Insta, there could be a rule to ban uploading images to Direct of <18 y.o. users, only reposts, meaning they are publically availiable and may get reported by other users and brought down by existing policies without breaking E2EE.