Probably because phones are rebooted so rarely - it only happens on an OS update or if your battery runs out.
Oh, usually once a week, sometimes twice a day, the phone will do the rebooting for me. Sometimes when it's in my pocket, sometimes when I try to watch a video that's just too long. Maybe some kind kind of memory error?
What you said reminded me of this, because I had forgotten this infuriating rebooting. I wonder if this was related to the building or something else? This is weird.