Exactly.
Try uploading a Disney movie you don't own broadcasting rights to, and make your argument "Well me and my friends were talking over and laughing at it, so it's really more about what we were doing, and also we wanted people to see if they want to watch the movie". I think video games are an artistic medium that deserves respect, honestly, and part of that respect is that the owners and creators are the ones who decide what's acceptable use of their work.
I mean personally I think in a lot of cases that streaming a game boils down to free advertising because most of the joy comes from actually playing them, and modding games is ultimately just contributing to their overall value and appeal. But it's not the players who get to decide that, it's whoever made it in the first place, and for story heavy games I could absolutely see "Watch someone else play the entire game start to finish" not being something they want to give away freely.