Actively playing games requires concentration, thinking, planning, quick reaction etc. and many games include long stretches that test your tolerance for frustration. Meanwhile, game streams can be a completely passive activity that add an illusion of company and let you stuff your face with snacks.
Plus, there are certain games that I suck at that have decent stories. And some games have decent stories but practically no gameplay.
Plus, my gaming PC is on my desk, which is not as comfortable as a couch or bed.