It's as trivial to sign fake content as real one, and people are really, really, really bad at anything close to due diligence for signature verification and revocation (as that's an objectively hard problem).

Furthermore, there's the catering to the least denominator. Whatever signing ability is available for a random widespread cheap third world smartphone used in Tik Tok videos will be treated as sufficiently good to assume it to be true; so if a determined attacker wanting to create a misinformation campaign with fake videos can circumvent the security of that signing process (e.g. get a bunch of valid keys indistinguishable from these phones, then fake videos will have as good signing as real videos.