> Signal on Android uses your phone PIN, for some insane reason.

The reason is simple: 95% of people would just set-up the same PIN anyway.

In unrelated news: Password reuse is rampant: nearly half of observed user logins are compromised

https://blog.cloudflare.com/password-reuse-rampant-half-user...

It can be set differently.

Not on Android. You can set your Signal PIN, which is a recovery code for if you lose your phone and are locked out of your Signal account. You cannot change the lock screen PIN, which is the same as that of your phone.