The attributes are cryptographically signed by the provider. With their public key you can check if they are actually signed by them.

Yes, such systems are in development and are called identity wallets. https://yivi.app/ for example has the idea of zero trust attribute sharing. You can request attributes the government knows and store these on your phone. You could then share an attribute like "over 18" with the porn site without the government knowing you shared it with them. Most identity wallets don't want to touch the porn industry tho. So it isn't used for that (yet).