I was once heavily afflicted by NIH, but with experience I realized how much time I was wasting stubbornly trying to reinvent the wheel. That’s actually a great analogy if you think about it—-the original wheels were fashioned out of wood and are quite simple in concept, but creating modern wheels require s tons of specialized tools and knowledge to build. It doesn’t make sense to attempt this if I can just buy a premade one that suits my needs. That premade wheel also has the benefit of huge amounts of iteration in response to problems encountered over time, and I would probably need to replicate at least some of that in order to build something competitive with the existing solution.
In the realm of code, using other people’s solutions when available lets me focus on the original problem I want to solve. It also allows me to minimize the amount of code I need to actually maintain myself, since typically there’s a group of people that are collectively much more knowledgeable than me doing that for free. If I ever do have a reason to know how the library works (patches, bug fixes, behavioral questions, curiosity), I’ve found it much quicker to figure out how the code works rather than write my own.