Because that costs energy, the fact that your CPU is idle most of the time is why you can get hours of battery life.
If I didn't have to worry about my grandparents getting scammed because they were having 99.99% of it effectively filtered or warned about at one layer or another before it actually became a problem...can you imagine how much you could lower that type of fraud/abuse?
Exactly! Apple for instance already does this with some ML tasks that only run when your device is plugged in, I think it's a great compromise.
Some thermostats prioritize low-usage times, too.