I believe liability is an issue too. In aviation (as compared with autos) the manufacturer is often found partially liable for an accident, so they can be on the hook even if it was mostly the fault of the operator.

It's worse now, people are doing "51% Build Assists" (most famously Vans and Sling aircraft) to save money where the factory would give you an IKEA plane kit and you will maybe put in a few screws and decals and install the avionics panel and you will sit in the room and watch them assemble the rest of the plane "under your supervision". Then the whole thing will get penciled whipped as an Experimental Owner-Built aircraft and if anything goes wrong it's entirely the fault of the owner and not the factory, because the layman hundred hour PPL pilot is supposed to be a master of aerospace engineering.

I think MOSAIC is the only thing the FAA got right in the recent years when it comes to general aviation.