The problem for a lot of users is the disconnect between a logical style (the actual boldness of the text) and representations of that style. If you haven't learned to handle this kind of abstract symbolism, plain text markup formats just don't make sense to you. Your brain instead registers it as a combination of "this editor doesn't have bold for some reason" and "the computer garbled my copy and made parts of it bold for some reason" without ever drawing the connection between one and the other.
This is only really an issue in places where abstract thought is seen as an innate ability which can only be tested for and not taught to people. Unfortunately this includes the US.