Open-source projects still have to sell themselves to get users (marketing/"devrel"). curl's author definitely makes money from curl: https://curl.se/support.html
Everything is an attention game, if people stopped using curl, then it would stop getting donations, and the author(s) would probably have to stop working on it as much. They aren't being "greedy" or anything, but they do have some level of incentive just to have a reasonable living working on curl.