Because you start with the absurd assumption that remote access is the primary function of a door.

It does not seem unreasonable that a users might want to open the door while they are coming down the street or turning into the driveway, before they are within wifi range. Maybe their home wifi just barely reaches the inside of the garage and doesn't extend past the garage door. Most folks are not exactly wireless network experts.

I agree it would be ideal to use a local network path if possible, but given that remote access is a requirement, I can understand why they just made it the default.

This was solved even before internet or wifi existed.