It just amazes me that literally everything has to round trip to the cloud for everything. It's like we forgot that things can connect to each other.
I believe internet is needed because tesla acts as a CA for your phone. That allows you to revoke a device, for instance if it were stolen. My guess is that the phone has authorizations that it renews fairly frequently and in this case apps could renew auths so once the current set expired you couldn't use the phone to unlock, etc. I can't immediately think of a better solution to the problem that allows using a phone for some functions and being able to remotely revoke a device.
If letsencrypt went down we'd similarly see a large portion of sites be inaccessible through browsers after 90 days even though you could directly connect to those servers
Having every car aggregated at one cloud system means if you got into that you could simultaneously attack every single Tesla in the world.
There is no good reason for this. It's just a mixture of "how things are done nowadays" and superstition.