If you upgrade to a broken OS version, that affects a massive number of things you do on your system including possibly your livelihood.
If you install a broken game, the issues are almost always completely isolated to that single game, and games are things you can live with being broken.
People want to watch a movie while it's still new, which is why theaters want up front exclusivity. Songs are "hot" for a while. TV shows are better when you're watching it around the same time as a lot of other people. Sure you can watch Breaking Bad for the first time right now, enjoy it, and have some good conversation about it with a few people. But it was different when every week half of your office or classroom was talking about the latest episode.
I still don't recommend buying games on launch date most of the time because of the current state of the industry. But it makes sense to me why people do it.