Probably because when you are learning Java it is seen as something that
isn't worth getting angry about as there are other more important things
in a developing Java programmers life (like learning how to refference
Objects :P)

Then when they start getting Intermediate level coders nobody is
checking small things like that, they are after more complex ideas, and
eventually nobody else looks at their code.

I would agree that everybody try and use StringBuffer as much as
possible, but I know when I am just Hacking together something for
myself I often find the append nature of StringBuffer can be harder to
read then than concatenating strings together :)