Multisig requires user cooperation, many users will not care to cooperate, and chain of custody verification really starts working only if you can get (force) ~100% of legitimate users globally to adopt the system.

Also, for the potential creators of political fakes, such a multisig won't change things - getting a manufacturer's key may take some effort, but getting (and 'burning') keys of a dozen random people is relatively trivial in many ways - e.g. buying off of poor people, stealing from compromised random machines, or simply issuing fake identities for state-backed actors.