Obviously there are a variety of sizes and configurations, but if you assumed 3.5MWh per 20ft container, that's 5771 containers, and if you assumed a PCS container per two battery container, that's 8656 containers, just for an idea of the scale.
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.