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.
I think MOSAIC is the only thing the FAA got right in the recent years when it comes to general aviation.
Some people dont want to pay the extra overhead and cost burden of the manufacturer taking partial liablity. They’re willing to take a risk.
Who is safer? The person who "self-built" (but actually allowed the trained factory folks to build) a Kitfox with a full glass cockpit, modern engine, and dual FADEC, or the person who buys a 1972 Cessna Skyhawk with steam gauges?