I think that people do it for a whole bunch of different reasons. Helping others is one of them, yours is another. But I remember that back at the start of code-sharing, the usual reason people gave for doing it was to help advance overall knowledge.
I think you’re being a little too altruistic. When I publish code, it’s not about helping people grow. It’s so that others can see the code they are using and that if there is a bug or mistake, they can correct it. Hopefully they share that fix back, but it’s not required.