The checker just has to have roots tracing to timestamping CAs. I really don't understand why Mozilla designed their system like they did. Code signing is well

The checker just has to have roots tracing to timestamping CAs. I really don't understand why Mozilla designed their system like they did. Code signing is well known and probably even done for the Windows installer of Firefox, why did they just not duplicate the model? Checking expiration vs current time makes absolutely no sense for code signing, esp. at runtime (it could kind-of sort-of make a little bit of sense at install time, but I'm not really convinced, and maybe it is even not really possible to distinguish between the two with an effective boundary)