Engine like Unity may actually have so many more features than we can ever imagine re-creating. On top of that they have Microsoft, Google and others as platform dev partners. Direct support for VR, latest graphics card features and so on. Same project running on Win, Linux, Mac, Android, iPad without code changes... At best the own engine is a thing that helps participants to learn about game engines, but it's like tasking 10 people to build a skyscraper when it comes to engine fitting for modern games.

A while back we released this Unity game https://www.youtube.com/watch?v=_fe2KDt7MYE , which had 1 artist, 1 level designer and 1 other programmer besides me. Physics, animations, graphics with shadows and all were trivial to do compared to how i could even imagine accomplishing on a custom made engine. Even something like making a skeleton explode and bones drop on the ground colliding all over the place. And no, nxPascal i code is nowhere near capable of doing any of those. What i really liked about was the animating that's really powerful. The engine allows you to write your own shaders, but what's even more impressive is the user support of thousands of people whenever you don't know how to do something by yourself. You get answer within minutes in most cases.