Because I have glasses the nearly thickness of coke bottle bottoms and no problems with small text, because I didn't wait decades for high-res displays that don't flicker just to throw the resolution away again, and mostly because I don't really look at individual letters as glance across them when reading a normal text with known words, so for me smaller fonts allow scanning larger chunks more quickly, and parsing words and phrases in chunks as well, while moving the eyes back and forth and scrolling a whole lot hinder me from simply absorbing the text. In short, because it's more efficient for me and there are bound to be people for whom it's the same. Because designing for my own actual eyes strikes me as honest or something.
