Privacy

Australia pressed it, as a result of parents who have genuine concerns about social media addiction in kids (but who aren't willing to do anything about it themselves).

And there's also this which explains why the implementation is the most annoying way possible. If I had my way age-verification would be solely tied to the addictive features of social media, but if that were the case the big tech companies would just lose users and wouldn't be able to insert themselves as an age-verification middle-man.