I suspect the aversion to a more blocky shape (higher volume/surface-area ratio), has to do with heat dissipation, which if I recall correctly, improves with surface-area.
EDIT: hence the flatter/thinner batteries we see in EVs.
That actually mostly doesn't matter for most batteries. Most EV batteries are actively cooled/heated with fluid moving inside. So the heat dissipation occurs in the radiator for the battery, not at the surface of the battery. We see flat batteries because sticking the battery at the bottom of the car helps with weight distribution and handling.
EDIT: hence the flatter/thinner batteries we see in EVs.