Sure, it’s free and the author can manage it however they want. But the reason they choose not to is because it’s still falls into the enterprise software envelope even if it doesn’t command an enterprise level price tag.
Just because something is released for free it doesn’t mean the developers don’t take their own software seriously. And there’s absolutely nothing wrong with them doing so either (even if you personally would like your software to have all sorts of unexpected behaviours and side effects).
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
I'm not 100% sure why you're being downvoted. While the choice of the word "sell" kind of feels at odds with FOSS, it's honestly not an inaccurate verb.
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.
curl isn't sold.