Unfortunately there is an incentive problem which means that library authors tend to introduce unnecessary complexity into their libraries in an attempt to cover more use cases in order to get more adoption. So many libraries tend to become "jack of all trades master of none". Also, as a senior dev, you might realize that a lot of libraries are poorly written and overengineered and that because you only need them for very specific purposes, it can sometimes be easier To write them yourself and save yourself a dependency.

And also a concern about dependencies is thay they make your project more vulnerable since you end up trusting one more person.