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.

Yeah, that's probably part of it. Some do have explosion vents in the top of the container.

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.