I guess some people were taught that the Not-Invented-Here Syndrome is a bad thing and needs to be avoided at all costs. And they took it literally.

I had a similar situation at a former job once, where as a subtask of some task we basically needed to implement a "partial deep copy" for a few specific classes (deep-copy some selected properties, shallow-copy the rest), which could either be implemented in ~50 lines of Java code, or by adding an extra library that provided this functionality using a domain-specific language, plus many other things, some of them potential vulnerabilities... and it took a lot of time to convince my team leader that "reinventing the wheel" is the right thing to do in this case.

"Why do you want to program something that already exists? Don't you realize that every line of code you write is a line of code someone else will have to maintain in the future?" Yeah, good points, but it's not like using a library is without costs either: you need to scan for vulnerabilities, increase versions, sometimes the API changes; in long term that is a lot of work to do just to avoid writing 50 lines of code.