But there's a difference between working on a product that impacts millions of people, versus noodling on a github project with zero followers.

Sure there is. When millions of people use your software you have to consider what those millions want. I guess it depends on what you enjoy about software development. For me I enjoy the craft itself and only let users in because I have bills to pay. My best work is stuff nobody will ever use or care about. It's like being a pop artist VS a relatively unknown experimental jazz musician.

Many finds value in doing stuff other people wants. Work is the most efficient way to do that, you write something and now millions of people can use it. When you write your own pet project nobody cares about it is depressing in comparison.

Of course you can care about other things, but if you care about this thing then work is the easiest way to fill that fix.