It has nothing to do with literacy, the em-dash simply is not on the standard US QWERTY keyboard. This means that people who purposefully use it, either have to copy-paste it from somewhere or (if they-re on Windows), use "Alt + 0 1 5 1". This is very obviously not a natural behaviour that 'literate' people use when they write.

It's not just less literate, it's also people who feel the need to be amateur prosecutors.

It's the same thing as judging people who wear their hair too long, or wear pajamas on the plane, or who wear pants that are too baggy, or who have children out of wedlock, etc. Some people are deeply convinced that society is on the decline and that they have a mission to ensure everyone else stays in line.

It's been that way throughout history.