[Friedduck]

According to WHO only about 4% have a visual impairment of any kind, and how exactly would one design for a dexterity impairment? Simpler to navigate menus?

And as wfjackson notes below, the browser statistics appear to also be in question.

Finally, I'm sure that someone would contest the claim, but it's far more difficult to design for blindness than browser idiosyncrasies. The entire UX has to be re-thought down to the last detail. When you consider the myriad interactions that could occur and the difficulty of building & testing for a small audience, it's no wonder that it's still the status quo.