I write code for a living and for my own projects, and I open source the code I write because it costs me nothing extra to do so, and I learned a lot from open source when I started programming so I want to give that back to others too.

Some of my libraries gets used by others, most do not. For the ones that do, I get some bugs reported and if it impacts my own projects, I try to work in fixes. For the ones that don't impact me, I reply to them that a PR is welcome, otherwise it's unlikely it'll get fixed. For the PRs I receive, I mainly look at the code quality and understanding if I can maintain that feature in the future. If it's too big feature, I ask them to fork instead, otherwise I try to merge it.

In the end, the toll for me is nothing of concern, so why wouldn't I open source it? Money has nothing to do with it.