I think Not Invented Here syndrome is generally foolish and quite arrogant, but Not _Implemented_ Here syndrome can be a reasonable default. You rarely truly understand something unless you've implemented it yourself. You can draw inspiration from other sources of knowledge and in the process of implementation, you may decide that the source is better than what you would come up with. You might also discover that you can improve on the source, at least in some dimension. Your implementation could be as simple as a wrapper around the api of an existing library (or a DSL for using it). But after you've written the wrappers, you may find that it isn't too bad to just implement the darn thing yourself (probably don't do this with crypto!). I find it is generally a mistake to just bring in dependencies and directly build on top of them because then you become vulnerable to bit rot and vulnerabilities (or just needless complexity the use cases of other library users that are irrelevant to you) introduced upstream no matter how conscientious the upstream authors are.

Only if you’re unaware that “anyone” also includes yourself.

That is more cargo cult behavior than NIH