Probably because the transfer of accounts (typically for reasons of better spamming, but in this case for adult access) is possible.
However, that makes me wonder what mechanism might "unverify" an account holder's age upon transfer. I suppose it's simply a need to re-verify (take a new photo) upon every login, but then folks could transfer the session cookie to avoid needing the new owner to perform a login (unless a new device ID/fingerprint makes the old cookie useless).
> … I suppose it's simply a need to re-verify (take a new photo) upon every login …
Clearly the only foolproof solution is a 3rd-party camera pointed at your face at all times whenever you use a computer.
10 years was enough for Tom Fulp.
SOTA is age inference: The platform studies your behavior to estimate your age.
Since you don't have to verify every time you use the account, transfer of verified accounts will still be a "problem" though. It's just a CYA to be able to say "we verified this account owner."