I'm personally kind of amazed it hasn't already happened, either through intentional (mis-)tinkering or natural mutation. The reason I'm surprised is air travel. People fly everywhere, and anything that appears that is easily transmissible ought to spread like wildfire.

It means one of several things:

(1) Humans are more resilient against plagues than we think.

(2) It's harder to produce a super-disease than we think, so it's a very rare event.

(3) We've just been lucky as hell.