I think a lot of this has to do with the online distribution. We are used to installing things downloaded from the Internet without much thought (of course there are security considerations, but that's another topic). On the other hand, new OS versions used to be special: you go to the store, or you order it online, it comes in a box, and there used to be an ominous feeling about it. It may wipe all your data, it may make your machine unrecognizable, it may kill your pets.