There is already a time signal in the electrical grid:

https://www.meinbergglobal.com/english/info/time-synchroniza...

> Why don't they just always use this Tech. I don't understand.

The bit's that get your oven to work don't need the time. Adding the bits for this is a cost and a thing that can fail because it can't receive the radio signal that equals a Dead on Arrival for your oven and a service call at least. When it's on a watch it's likely near your wrist which is likely near a window which likely means it can get the radio signal to sync.