It may seem simple to write leftpad, but if 1000 projects that need it all write their own version, there will be at least 2000 more software bugs out there in the wild because of it. If you think that's rediculous, you're not being realistic about the huge disparity in skill levels of industry programmers as well as the considerable rush under which many projects are done.
Also important is that every time I can use a public ally available utility instead of writing it myself, it's one less thing I have to think about. The benefit of making less decisions really adds up, saving a ton of mental capacity to focus on the more important parts of my project. Even the simplest methods require some thought to design.
I know there are disadvantages (such as what happened as the topic of this post), but there are also ways to mitigate them. As far as having many versions that all do the same thing, there is usually winners and losers over time. Because of this I believe that eventually the dependency graph shrinks overall.
Note that I wouldn't advocate creating utilities for things that are not very generalizable.