Proven designs have already been made, the numbers are well known. Different tips rudder canopy to make it their own. Engineering the kit and marketing is what they do with ease, not my job. Just don't know why they're wasting time regurgitating some generic beginner plane.
Dan has it - because they are in business to sell airplanes. Take any established design and make a kit. For the next 20 years people will argue over whether they should have moved the bellcrank pivot 1/4" to the side, and endless arguments over whether it is a good design or not. Or suppose someone finds something slightly new/different - now their kit is glued to the shelf, no one wants it because there is something microscopically "better".
It's an even bigger issue with engines. ST60 sales were through the roof, along with the price, from 1985-1988. But, suddenly, it was completely obsolete and you couldn't give them away. Why do you think Windy, et al. went nuts over it? Local example - "we" were running 40/46VFs and the glow plug to have was the Glo-Devil #300. Hobby shop stocks up on them. Then, we find out that the Thunderbolt 4-Cycle plug gives marginally, possibly, slightly more "power" (not to mention half the price). End of Glow Devil #300 sales forever. I am sure there was a peg full of them in J&M Hobby house when it closed.
RSM can do it because there is low overhead and the kits can be made on-demand. There is tons more to marketing a kit to a general hobby market than most people can grasp.
Brett