Not true. You might not want to make money yourself but you also might not want other people to rip your work off and profit from it.
People that open source things do so as a gift to the community at large, and sometimes people just don't want to give away their property (intellectual in this case), even if they don't intent to turn a profit. Plus, leading an open source project can be quite stressful and there's also the time cost of actually supporting a community of people, if they gather around a project.
There are tons of downsides to open sourcing something but occasionally there's a significant benefit (or other motive) that drives the decision to open source and outweighs the negatives.
