Why aren't these popular?
Too much time, effort and money (up front and ongoing) for what you get.
Mainly, limited performance, weights and durability......
Ownership costs, hangarage, maintenance.
Pay double or tripple for a plastic fantastic and get 10 times more capability at very similar ownership costs and with a 4 stroke better fuel efficiency and reliability.
You still need to maintain a pilots cert and bfr for both.....