Back when there was no commoditization of game engines, every team starting a game had to do their own engine, tools etc.

Now that you have unreal and unity etc, you can actually concentrate on game design and gameplay instead of worrying about the engine and tooling.

I agree though writing a game engine and physics engine is very interesting. I used to work for the R&D team of a casino games manufacturer. We created our own in house game engine. It was fun, but adding any new feature would take a very long time. In the end we shifted to unity.