Because you could ask for a checksum of the firmware and if it doesn't return the right hash you know something isn't right.

You also never store the hash, so once a user has gained access to the car it's impossible to get the right hash (as you would've had to modify the firmware/filesystem/etc in some way to gain entry).

You would also need to include a timestamp/car serial in the hash so that you couldn't reuse an old hash from before your entry (that you had MITM'd) or use a hash from a different car that still had its integrity.