Can't release just plans. Almost all kits don't have sufficient information on them to build the model. You'd have to do some of your own design work, so to speak, to create the parts not shown on the plans. Many times, only root ribs are shown on a tapered-wing plan, and not the full complement of ribs, so you'd have to loft the rest of the ribs on your own. Most fuselage bulkheads aren't shown. Many of the smaller parts aren't shown. You would not only have to be skilled at reading the plans, but also skilled at creating the parts you need by inferring their shapes from the plans.

So to release plans sets would require re-engineering the plans. Because people won't pay that much for plans, it would be very, very difficult for plans sets to sell at even a break-even pace.