Its a lot of cost to come up with all new tooling for beams, mast, rudders etc. If you are starting from scratch and don't have any tooling then you have to make it anyway and so why not make it light but if you already have the beams, mast then just use what you have. As for the hulls, not sure what they weigh but its a tradeoff of cost vs strength vs lightness and you can't have all three.