Comment by EdgeCaseExist

That's kind of an engine though ? An engine is just a bunch of low level types, DS, algos that go onto build mid level abstractions that go onto form systems. In this case you can argue the game itself (aka the runtime) is the game engine?

As others say, getting data (Assets) in/out of the engine is the hard part. Especially as they are the real contribution to the shipped game been so large (unless smart compression, delivery optimisation is used, sadly not common).