That's not the case though - why would I reinvent the wheel when I have finite time to get something done? I love to suss out solutions to all sorts of things on my own time, but if I have to left pad a field, I've already got lodash, I'm using that.
In addition, when someone goes back to read the code "_.padStart" just makes sense to read.
I'm not saying that understanding doesn't lead in repetition and effort, but I think you're wrong on why people use libraries. It's not because people don't want to know, they gotta get whatever it is they are working on done.
Might as well shortcut all that and see if there is something in standard library or in an existing package...