"Why not use [Example OSS Library]?"
"Nah, I can do it in 15 minutes."
"You know, they have [X] contributors, lots of great documentation, and 6 months of work into it."
"Yeah, I know [but whatever.]"
Depending on the situation, the library may have a ton of extra bloat that your application will never use. Also, recklessly deferring to open source libraries can potentially snowball into something like the left-pad fiasco[1].
[1]: http://blog.npmjs.org/post/141577284765/kik-left-pad-and-npm
It depends on many factors:
- what is the problem? (some problems are really hard, some problems are trivial and just seem hard)
- is developer experienced enough to build own solution?
- are existing solutions good or badly written?(I often refrained from using existing solution when I looked on bloated code in it).
- is solution suitable to project? ("too much bundle size", "doesn't support X", "not too configurable", "too slow", "too hard to use").
etc.