I've always wondered why they don't stack them in deployment -- the containers are obviously fine with it! -- but I guess land is cheap enough not to bother.
Horizontally they need plenty of room for airflow, as each container has some HVAC-or-whatever module on the side. But vertically...
Foundations, maintenance, fire risk.
Land is cheap relative to a container full of cells.
They are pretty heavy and batteries already need to do adequate foundation work before installation.
I would assume it has something to do with fire hazard ?
I always thought they were spread apart so much to further reduce the chance of catastrophic spread of fire from one container to another.
That would be a lot harder if you stack them since heat likes to rise.
But no clue if that is the actual reason.