You'd be surprised how much code people blindly reuse without even looking at it, especially in JavaScript. A bunch of it is from projects owned by random individuals. The JS standard library is ridiculously small, so nearly all JS apps import third-party code of some sort. One JS framework can pull in hundreds of third-party modules.
It's much less of an issue with languages like C# and even PHP, where the first-party libraries are often sufficient for building a small or mid-sized app.