Why is the vision correction in VR headsets only an afterthought?

Researchers predict that by the year 2050, about half of the world's population will have myopia.

Considering the target demographic, a significant number of potential VR users suffer from myopia already. Why are there no more VR headsets with adjustable focus?

Several vendors offer replaceable lenses, or various addons to fit the glasses in, but the obvious solution used by the early cheap headsets like GearVR - adjustable distance between lenses and the display, is not being utilized for some reason.

Is it a technical problem, economical problem? Are the modern lenses somehow tuned for a specific distance?