they don't. I mean for example Amazon puts all new hires on "on call" status for like a week every month. the LAST people I would want working On Call and waking up at 2am to try and solve something are fresh grad hires. You can actually watch videos on youtube of new grad amazon hires doing this, they actually document themselves, and the vast majority of them are "well it's 1am and I just got a call...I'm going to try and fix this ticket but really I have no idea what I'm doing" annnnnd generally nothing gets fixed or they break it worse. So they end up being sleep deprived, going into the office the next day and sleeping at whatever workstation they can find available and it leaves you wondering "what's the point?"

I personally am of the belief that being on call for stuff like this is pointless when you're world wide and could literally just transition the stuff to a different team in some other part of the world but I guess Amazon treats it as a sort of initiation process or whatever.