Some of that may also be coder preferences—someone makes their own libraries or framework, because they don’t know or don’t like what’s already available.
For example, I intentionally recode my site from the ground up every 18 months or so, to test & update my knowledge. I primarily use HTML & CSS. (I used WordPress once, and I HATED it. It kept editing my written-in-HTML pages, and it was harder to play with the FTP side of things.) I’ve looked at Bootstrap, to see about doing one of my site iterations using that, but I cringe within seconds and go back to my personal templates—which are waaay more efficient than Bootstrap, which also isn’t naturally compatible with the way I design sites.
As my fluency increases with JavaScript, I wouldn’t be surprised if I do something similar with or two those libraries. So why wouldn’t already-fluent folks already be doing that?