Need to do X? If you write your own library that does X, chances are you'll be the only one to ever work on it. Need a new feature? You have to stop working on your actual project and implement that feature. Found a bug? No one else will fix it for you.
If you depend on a library that thousands of other people also use that does X, if you do find a feature you need that it doesn't have, open a ticket, someone will likely do that work for you. More often than not, that feature already exists, all you have to do is read the docs. If you find a bug, report it and and wait for it to get fixed, but there's also nothing stopping you from fixing it yourself and submitting a pull request.
I agree with most comments here that it's not a good idea to reinvent the wheel everywhere, but when all you want is a wheel, and not the entire car, it's ideal that we have a way to just include the wheel.
Classic case of NIH syndrome.