Getting things for free isn't just realistic these days, it's genuinely become the norm. Competition is infinite and there's always somebody willing to bleed VC capital to offer things for free until they finally attempt proper monetization and everyone jumps ship to the next one that's earlier in the cycle. The one unicorn that does successfully monetize then subsidises all the rest through further VC funding in a roundabout startup UBI of sorts.

Besides, Stackoverflow runs ads and profits directly from users' posts. You don't get to own things just by the virtue of being a platform that lets people host things. Does Github own all the people's code they host? Does a cloud service own the software people put in their containers? Of course not. Stackoverflow themselves state that each answer is the property of the poster, licensed as CC BY-SA. Even Reddit's TOS states that "You retain the rights to your copyrighted content or information that you submit to Reddit".

Now whether there should be a class action lawsuit including everyone who's ever posted on Reddit or Stackoverflow against OpenAI is another story.