If a CA gets compromised, the attacker can impersonate anyone. If instead you loaded up your certificate with loads of signatures, you’re no longer relying on any one organisation or government’s honesty.
Certificates could also contain statements of intent like ‘I plan to use certificates signed by at least 3 of the current signatories for the next 24 months’ or ‘I implement delayed certificate rotation so assume this certificate is compromised if it’s less than 24 hours old so don’t use this if I’m not in a CT log’