Constant-section wings allow the use of simple flat-wrap skins, identical ribs, and simple spar web/cap parts. They can also be relatively easily shortened/lengthened. Kit manufacturers like them because they reduce tooling costs (fewer forming dies), inventory costs (fewer unique parts to stock) and allow easy reuse of parts/assemblies on multiple models. Scratchbuilders like them for the reduced tooling costs and the ease of fabrication.The Market likes Hershey-bar wings.
You're right that it's seldom an optimized lift distribution priority.
Builders like near identical ribs and nice single curvature surfaces. Hershey-bar wings also have nice predictable stall characteristics.
I don't think it has anything to do with the designer being lazy or ignorant, It's just market demand.
I believe I heard the same thing. The RV-12 is very much optimized to be right at the LSA limits and still be able to take two average-ish sized Americans. It wasn't an easy task.There's also a weight difference, one-piece flaperons can be lighter than two separate control systems. I believe I read a story about Van designing the RV-12 and just about pulling his hair out trying to save weight without weakening the primary structure. Saving 3 or 4 pounds of control runs and pushrods might have allowed him to use a .020" thicker spar cap, or the next size up in D-tube leading edge skin or something?
The -12 also has wings that are designed to be removable. Not many owners/builders wind up using that capability, but the flaperon configuration also makes that a little easier to implement (one control connection to worry about instead of two).