> You might also have a valuable mental toolkit that is squandered when using an existing engine. I can’t count all the times I’ve said, “I know how to implement that, but not in Unity.”

Isn't this the number one reason why people reinvent the wheel? We all understand things differently. The people who make the libraries and frameworks have their own design and developers are supposed to use their code in very specific ways. Deviating from this path causes a lot of pain and difficulty. Sometimes it feels like it's easier to just write new tools based on what I already know compared to learning a completely new set of ideas just to work with existing stuff which may or may not be adequate for my needs.