I have a tendency to prefer writing my own library for something instead of relying on someone else's. Not always, obviously, but quite often existing libraries fall short or are too complex to use. And solving the problem myself is almost always an educational experience.
For example, I'm currently writing my own graph layout library because I'm not happy with Dagre and ELK.
For personal projects that's fine, but I'd absolutely hate to work with someone who is rolling their own libs for everything, and then when they leave the company puts an enormous amount of tech debt into our hands. It also really doesn't make for good team-work if a developer always forces their own opinion on everything.
Also, your own library, at this point, is much less of an attack vector than some dependency from a package manager