Is it only because there's US East and US West? I.e. Maine and New Hampshire can share power generation, and Arizona and New Mexico can share power, we just can't share across the East/West divide, wherever that is?

Correct, because the east and west grids are not synchronized you can’t really transfer power between them: https://en.wikipedia.org/wiki/North_American_power_transmiss...

> The Eastern Interconnection and the Western Interconnection are the largest. Three other regions include the Texas Interconnection, the Quebec Interconnection, and the Alaska Interconnection. Each region delivers power at a nominal 60 Hz frequency. The regions are not usually directly connected or synchronized to each other, but there are some HVDC Interconnectors.