Custom Game Engines: A Small Study

I feel that people today overestimate how difficult it is to create a game engine, or maybe they underestimate the other aspects of game development.

For clarity, I mean here a basic engine (for example, a custom engine for a 2D game), not a photorealistic and generic 3D game engine like Unity (that definitely is very difficult and probably not worth reinventing the wheel without a good reason).

Creating a custom engine is a big endeavour and not many are ready for that. I recognize Unity (and Unreal to less extent) have really democratized video game development. Lots of small-medium size companies can use Unity today to develop games quickly, and sometimes, with very low budgets! But, still, lots of big companies prefer to rely on their own custom technologies.