This might be fine for some utility functions which you can tell at a glance have no errors, but for anything complex, if you copy you don't get any of the bug/security fixes that upstream will provide automatically. Oh, now you need a shim of this call to work on the latest Chrome because they killed an api- you're on your own or you have to read all of the release notes for a dependency you don't even have! But taking a dependency on some other library is, as you note, always fraught. Especially because of transitive dependencies, you end up having quite a target surface area for every dep you take.
Whether to take a dependency is a tricky thing that really comes down to engineering judgement- the thing that you (the developer) are paid to make the calls on.