Why does google decide to log people out of all their google accounts all at once with no warning?

I have a couple (e.g spam, uni, personal) and it's so profoundly unhelpful to make me sign into them one at a time by coordinating between 2 or more different devices. an occasional spot check for security would make sense if I was just booted out of one account, but instead it logs me out of every single one at once?? What gives?