I just want to do my job and nothing else. Why must I always aspire to get a promotion?

Sure it’s more money, but it’s also more work. Why do non-profits in particular seem to want you to move up or get out after a few years? I like my job where it is because it’s enough work (that I enjoy) and it allows for plenty of personal time to do the things I like. But, I’m feeling pressured at work to constantly be doing more and expanding. Why?