I might say that if you tried learning it then you'd realise that it's very difficult, but the real reason is that in most cases it's just not worth it. Virtually useless to know outside of the country, and if you're inside you either know it well enough or you have a job that doesn't require it (like all the English teachers and programmers) and enough support to get by without it.

I say this as a resident who is grinding his way through the language daily because I want to, but I am aware it's really a bit of waste of time like any other hobby.