A lot of people don’t understand that the Eastern, Western and Texas grids all operate at slightly different frequencies, they are non synchronized.

This is the same reason you can’t plug a gasoline generator into your wall outlet and not blow a fuse when you are connected to the power grid. The phases will conflict.

Residential solar connected to the grid uses a specialized grid – tie inverter which ensures that the output of panel matches the sine wave exactly.

So it’s impossible to move alternating current between the different grids. So it’s not like they can just build ordinary power lines to connect the seperate grids. They do move some power between the grids using converted high voltage direct current but that’s expensive and complicated to do, so it’s only done occasionally.