alt Hacker NewsI started out writing my own engine. Never finished a game. Switch to unity and finished two then hit industry.
Both games were mediocre if I am generous.
The janky demos i made with the custom engine felt and looked so much better. I even had folks on forums asking if they could join the project.
The issue is if everyone uses the same unity standard shader, lighting, posteffcts, etc, it ends up with a certain smell. The effort it takes to work around this requires implementing the version of the given components that better match the vision. Most custom game engines still use a plethora of libraries replacing the ones that matter. It also isnt so binary. Earlier box3d post was literally catalyzed by Unreals physics engine lacking some 3 line of code.
I think what you are really speaking to is that few people have the discipline or motivation to complete anything. The two games above that I completely were in a large part because i actually sat down wrote a design doc, made a schedule, and stuck to it aggressively reducing scope.
I recently wrote an article showing just how easy it is to make a game engine using css box shadows as the renderer, web audio, and a stupid simple tween system. I think it illustrates pretty well that people vastly overestimate what a engine is.