We do not have one giant grid in the US. There are several power pools, each with a different “phase angle”. People who work in power plants will know what I am talking about.
The only way to interconnect with theses power pools is to convert your AC to DC, then back to AC on the other gird.
I do work in the power industry...the USA has 3 interconnects as they are properly called. You are correct each has a different phase angle of their nominal 60hz frequency. It was attempted at one point to try to synchronize the Eastern and Western interconnects but the reactive power and system inertia was too great to bring each into.synchronous. Texas has ERCOT and the third interconnect which is also on it’s own phase angle. Texas could phase synchronize to either the Eastern or Western interconnects but not both. As it stands there are a few back to back AC/DC/AC tie ins between the East and West. Texas has five ties of 100 to 600 mw each and a variable frequency transformer of 100 megawatts in Laredo that is the only means of AC to AC transfer of differing phase angles.
The Tres Amigas project was to be in New Mexico as a 30,000 megawatt AC/DC/AC with a three way DC to DC tie allowing up to 30,000 megawatts to flow from any of the interconnects to any other one but politics shut it down. The blackout in Texas in Feb wouldn’t have happened with 30,000 megawatts of inport power on tap but Texas would have to abide by the feds rules and they refused it is why ERCOT doesn’t cross state lines to keep the feds via the commerce clause out. Having a DC tie of that level of significance will fall under the commerce clause and it was rejected.