It may be a logical default, but there's nothing "natural", and certainly nothing "sacred", about black text on a white background. Basically it's a socially accepted norm. Back in the early days of HTML different color text on different color backgrounds seemed almost mandatory. It's my guess that this popularity of anything other than black on white stemmed from the same sense of freedom that the plethora of fonts and colors in graphic word processors offered - a sense of freedom that didn't wear off quickly enough, but happily has long ago been relegated to the dustbin of web pages.

I think I've known only one person who when working within Word defined her default screen as white text on a dark blue background, even though many people claim that that's considerably easier on the eyes. It turns out, BTW, that back in Word 2007 this option was discontinued. Since then someone who wants to work in that way has to define that setting for each document separately. In Word 2013 the procedure is to choose "Page Color" from the "Design" tab, and then to set the color to "Blue, Accent 5, 50%". When we do that the text (if it's black) automatically changes to white. It works, but I doubt it's worth the effort.