TLDR: Meta want to push all the age verification requirements onto the OS makers (Apple, Google, everyone else gets caught in the crossfire) so that they don’t have to do anything AND they want it done in such a way that they can use it to profile people to push them targeted ads.
Its like they want to keep being seen as the bad guys.
I think this is also a way of getting ahead of any “ban social media for teens and preteens” bills that might pop up in the US. They do not want repeats of Australia! By adding age verification into the operating system they can deflect responsibility but also respond to legislators with a scalpel rather than getting sledge-hammered.