and then it's roughly just a rectangle of 100 by 100 containers, still big, but maybe not sooo big :-)
Horizontally they need plenty of room for airflow, as each container has some HVAC-or-whatever module on the side. But vertically...
That would be a lot harder if you stack them since heat likes to rise.
But no clue if that is the actual reason.
It'd just be impressive to see a giant wall of battery. A bunch of little batteries camping in a field doesn't play the same visually. Ah well.
Land is cheap relative to a container full of cells.