Which was the motivation for cryptographically attesting the boot process and OS, and in part paved the way for app attestation.
There are alternatives though: The Android Hardware Attestation API enables attestation on custom ROMs, but the attestation verifier needs a list of hashes for all "acceptable" ROMs. GrapheneOS publishes these but there's nobody, to my knowledge, maintaining a community list.
Cryptographic attestation is not a problem in itself, the problem is exactly what you already somewhat hinted at: it's who and how decides who to trust and who gets to make (or delegate) the choices. You can make a secure system that lets the user be in charge, but these systems we're discussing here don't (and that's by design; they're made to protect "apps", not users).