Work in support sometime. It is EXTREMELY common to find users that really don't know about right-click. Or that double-click everything. These are not new users either, they have been using computers for decades.

For new users I might agree that they could figure out a right click because everything is new. But for users where everything is familiar ("I've always done it this way"), it gets much harder to break those expected actions.

Second part is about explorer itself. While it might seem like a common utility (even necessary), I find it very rare for a general user to know about it (my wife does not, nor my parents, siblings, their friends,...but my wife's father does). Once they see it, they have no idea what to do. Copy a file? Really, they have no idea.