A game engine is really just a set of libraries. And making good libraries that compose well and are at a useful abstraction level takes taste, which comes from

A game engine is really just a set of libraries. And making good libraries that compose well and are at a useful abstraction level takes taste, which comes from experience. So code more! Could be hacking on games, could be doing other programming. But once that software design muscle is big enough making an engine will seem more obvious than ever.