Then you only know that someone is over 18. You don't know who is over 18. In particular, you don't know it's the person who's accessing your website.
that's the same situation as children putting in their parents details/scanning their face/whatever, you're not going to solve this technically
the point is non-govt entities shouldn't get any information about you during age verification other than that you're over 18 and that's what ZKP can give you