It needs to because otherwise supply voltage drops and you get brownouts.

There is no provision in light switches, outlets, thermostats, industrial machinery, or anything else to first check "is there enough current in the grid right now to run this?". They just draw current when connected, and changing that policy would be a huge endeavor.

Depends on what you compare it to. Less huge than replacing oil and coal?

Besides, you don't need to check for current, you check for prices. Which would be simpler.

This is called "demand response" in utility circles, and to varying degrees, has been operational for decades. Lately it is becoming much more reliable and accessible thanks to smart grid networks.

Thanks for the pointer. The Wikipedia page on the subject was quite extensive: https://en.m.wikipedia.org/wiki/Demand_response