For me when there is a library or framework or "engine". You are often having to learn the specifics of how this thing works, and ALSO map your mind to the way it works. When you write things yourself you are only in your own mindset. So you can write your 90% self implementation in less time than learning this new mindset. Then you can build everything together without special ways of piping things together. Adding more frameworks or libraries is an exponential explosion in complexity, and more and more of your time is spent with glue code. When you write everything yourself, you are in one design space and things flow more naturally.