>whenever anyone argues "why x when y already exists".

Yes, seen many people argue that, including to me, e.g. about some of my blog posts, which show hand-rolled ways of doing things when a library for that exists. I guess often such people are not doing anything much themselves, and don't get that people can create something in the same area where something else exists, for any number of reasons, even just for sheer fun, love of the field, or for teaching beginners.

or simply to have control over the code.

Yes, good point. When it is not a blog post but real-life project code, I sometimes do it for that reason you mention.