HVAC suffers from loss over distance. Large distances like what's between the western US deserts and the eastern seaboard would suffer large losses to heat via HVAC.

HVDC can solve this, but that requires an investment into this kind of infrastructure. Moving the batteries is using a preexisting infrastructure because the assumption is that new infrastructure won't be upgraded. We will build new so long as a ROI has quick turn around, another assumption here being that long term profit planning won't happen so everything needs to be planned to have profiting within two or less years. But we won't build new if usage of that new happens a decade later.

We could totally send the electrons over, but sending the batteries over is adding a bunch of assumptions that people won't want to do massive investments in basic infrastructure to facilitate that, so we've got run with what we have that can ensure profits in a fairly rapid pace before investors bore of it or the next election cycle tosses everything in chaos.