Oauth2, JWT's, hashes, timestamps, validations, and such, are all totally simple until they're not. The black hats have way more experience and way more time invested in this space than most any normal dev.
There are plethora of mistakes one can make in implementing AuthN/AuthZ, and many of them almost immediately will lead to either the direct leak of PII or can form the start of a chain of exploits.
Storing password hashes in an inappropriate manner -> BOOM, all your user's passwords are reversible and can be used on other websites
Not validating a nonce correctly -> BOOM, your user's auth tokens can be re-used/hijacked
Not validating a session timestamps correctly -> BOOM, your outdated tokens can be used to gain the users PII
Good enough for almost any startup in 2025.
Cryptography part is fine but storage or the auth process isn't.
You would like to think that no-one would write their app that way, but there are plenty of slightly less worse things that happen in practice and vibe coding probably introduces all sorts of new silliness.