I'd guess it has more to do with the fact that a big wooden head would be too heavy for the structure they have, and a foam one would blow over. It might save some costs to have a smaller head, but it can't be that much, plywood is $29 for a big sheet. Skimping out on one sheet over two or three would've only saved a miniscule amount, and I don't think we're at quite those levels of penny-pinching yet.