You've clearly never used a modern game engine; things have changed a lot in the past 20 years. Using a modern game engine and building a game engine are as dif

You've clearly never used a modern game engine; things have changed a lot in the past 20 years. Using a modern game engine and building a game engine are as different as building your own car and driving, or even designing a digital camera and being a fine art photographer. Both go really deep, but in entirely different directions. There probably isn't even a statistically significant overlap between people who are great at one being great at the other. The people designing and assembling the high-level logic for Zelda Breath of The Wild, for example, would get absolutely no use out of knowing how to build a game engine, and that's a good thing. When it comes to the way the game works from a player perspective, the underlying implementation details are invisible. Studios employ game engine engineers for a reason and they're not really using the engine. They're using code to make tools that other people use in the engine.