it appears their spars are just some sheet metal
A bloody great big ship is made out of "just sheet metal"
main chassis -rails on a lorry are as well! There's no reason why a casting shouldn't be used, especially if in a malleable grade....likewise a forging....or even a die-casting........but a "sheet" metal fabrication is often the most cost-effective, lightest and strongest solution to the need for a main-spar. Some lightweight aircraft
do use an extrusion...in the form of Alloy tubing, but that is probably because it's an "off the shelf" standard diameter,wall-thickness and material-spec and would be vastly cheaper than a custom-made section.