Well structured low level game code will lead (at least the most of the time) to pretty much what an engine would look like. Clean, reusable primitives.
That's the point. By actually writing games and sharing code between them, you will naturally end up with an engine that is proven to work. But if you set out to write an engine just for the sake of it, you are almost guaranteed to end up with some random collection of things that are barely useable.