I would guess strength has alot to do with it. Look at any tank that is meant to hold large amounts of pressure. They are all round. This creates a equal pressure on all 360 degrees of the shape, rather than at a weaker point such as a corner with a weld.
Plus the % of market that is taken up by lawn mowers is so tiny that it would cost a lot. The tanks are more sophisticated than a plastic injection like on a gas mower.
Exmark Viking 48"