Remember that you can arbitrarily tweak contrast with monitor and ambient light -- I'm reading tons of science papers which are always black-on-white and few minutes spent on setup made reading a pleasure. IMO the real problem is that the default setting in most devices is a mixture of inhumane brightness and grotesque oversaturation -- thus designers try to fix a virtual problem hurting people with a proper setup.
There's another reason that good design rarely uses the full range of contrast in any part -- because occasionally you want to go outside the normal contrast range for additional graphical "meaning".
So if your webpage is a light gray background with black text, you can still go even brighter in specific areas. For example, you gain the extra ability to use white to give a call-to-action an extra "pop", or to delineate page sections, etc.
If your page is black text on a white background, then you can never go outside of that, so your functional design vocabulary becomes more limited.
