In short: it would take me a whole lot more time to evaluate whether or not to depend on something as trivial as leftpad than to write it myself. I'm pretty confident I can do that in a minute or two and I trust my collaborators to not break it.
If you fixed the bad behavior you're experiencing, and the original author's effort saved you hours or days of coding, what's the downside?
Perhaps I'm not arguing for 15 second long code changes. But other than typing a single if statement, what takes literally less than one minute to securely change in any partially-complex project?