I can build a simpler level editor that I can give to a totally nontechnical person (a.k.a. my spouse), and they can go to town making levels.
They don't need to know anything about Unity/Unreal. They just need to know about my game.
My game already includes levels my friends have designed using tools I made for them.
I've been doing an engine on the side (c++ underneath, javascript on top) for pc,Hololens,magic leap,Mac, android, we , pi/Linux (finally) for about 4 years now. I can churn out tools (editors, daemons that run for months, hot-reloading remote-assets, vr apps&overlays, ML tools, streaming point clouds) and demos, but it's still missing rigid body physics and a lighting engine (outside ray marching)
I still have to resort to unity or unreal to get jobs out, because there's too much to do for one person. (Although this is finally starting to change :)