Maybe there are local bottlenecks inside the CISO network, but that's a very separate problem.
Also a gas turbine core that's spinning can modulate power in seconds, unlike one that has to first be brought up to speed with external power (even if that's from batteries, or an on-site diesel (like AFAIK in the case of the black start capability of the downtown power plant in Wuppertal Barmen)).
So it seems to me like you could power the US off california solar if they had enough.
The longest curently runing line is over 2500km, which from Los Angeles would cover the entirety of 17 states as well as cities like Vancouver, Calgary and even Winnipeg
There are four: Xingu-Estreito and Xingu-Rio for the Belo Monte dam, and the two parallel Rio Madeira lines for the Santo Antônio and Jirau dams on the Madeira river. They all take large amounts of hydroelectric power from the sparsely populated North region to the densely populated Southeast region. (There are also the two parallel Itaipu HVDC lines, but they're shorter than 1200 miles.)
Because the grid is, fundamentally, still storage-starved (though that's been improving in recent years), even with export the CA energy companies run into grid over-saturation problems on clear, sunny days. Eventually losses catch up with you pushing the power geographically far enough; they couldn't, for example, power the entire United States off of enough California solar.
So I naively suspect the issue is either exporting just saturates the adjacent grids too or other power companies are loathe to crash their prices by letting their grids saturate and therefore use whatever restrictions are allowed against exporting to keep CA's surplus off their grids.