Because it PARTLY determines whether to honk by whether the driver's seat is occupied. Passengers can get in and out all day with the car on, and it's fine. The driver can open and close that door all day too, as long as they're seated.
I learned earlier today that in the 2012 Volt I’ve been driving for nearly 11 years, the "fob outside a running car" alarm will sound even if the driver’s seat is occupied. I got in my car and pressed the blue button to start it. I then opened the driver’s window, and held the fob in my hand and stuck my arm out the window and held the fob outside the car as I opened and closed the door with my other hand while sitting in the driver’s seat. The triple honk alarm sounded to let me know the car was running but there was no fob inside the car. Didn’t matter that I was sitting in the driver’s seat.
I then got out of the still running car with the fob in my hand (ding ding ding because the driver’s door was open), reached my arm through the open window to hold the fob within the car, then closed the door. No alarm sounded (because the fob was inside the car). I then drew my arm and fob back out the window, and no alarm sounded when the fob was outside the car.
I don’t know if this is a "one time" alarm, but I suspect it is. That is, the first time the driver’s door is opened and closed when the fob is outside the running car, the alarm will sound, but any further opening and closing of the door with the fob outside the running car will not set off any alarm.
I strongly suspect that the driver’s door is the only door that will trigger this alarm. That is, if the fob is outside the running car when a passenger gets out and shuts the passenger door, there will be no alarm. Thus if you get in the car to drive your wife to work, and your fob is still in the house, but hers is in her purse, the fob in the purse of the passenger allows you to start the car and drive off... then when she gets out to go to work and shuts the door, there’s no alarm to remind you she is taking her fob with her, and now there is no fob remaining in the car, so if you stop somewhere on the way home and turn off the car, you can’t restart it because she has her fob and your fob is at home.