That is the point: the "drawing code" is perhaps 20% of a game engine.

You are missing asset management (loading, streaming, caching...), audio (big enough to split into libraries and actual audio engines), input (of different kinds), networking (big enough to split too), AI, scripting, physics...

The one who misunderstood is you. When people talk about a "game engine" in 2020, they are not talking about drawing code. They are talking about something much, much bigger.