Two reasons not to build in house:

1. Edge cases. You cannot possibly cover every possible edge case. This is why back in the day we had "Site works best on IE". Libraries are often used by a lot of people thus edge cases you couldn't possibly think about are covered.

2. Someone else maintains it. If your job is to build an application, why are you spending all your time maintaining a library that just facilitates your application? If I'm building a racecar, I'm not gonna make the wheels in house, let people who are better at that do that while I focus on what I do best.