The aim of reusable parts was to make software components behave like electronic components: purchasable from vendors, standardised, interchangeable, can be fitted together without conflicts. Nobody would consider replacing a NE555 or op-amp with its discrete transistor equivalent.

It sort of succeeded, but only at the data structure layer. Perhaps the high point of reuse is npm-style dependency-orientated programming, where people really do reuse tiny functions like leftpad(). But it's not stable. Because nobody is interested in providing that stability, and there's no money in it. There can't possibly be, because nobody will pay for tiny software components like leftpad. The market doesn't exist.

You can't second-source leftpad(), and that makes it possible for one person to break a lot of random builds across the world.