Because the majority of readers on the internet skim pages and whether subconsciously or not, they are picking up on keywords, headlines, links and the like. If you take the approach to simply "make the text #000 and the page #fff", you are simply making matters worse.
If they can't read your site comfortably than why should they spend more time on your site than it takes to close the tab?
See my other comment, the recommendation is usually #333 on #fff, or at least try to follow the WCAG standards:
https://news.ycombinator.com/item?id=7704533
When I see a site with such low contrast I think "the owner of this site doesn't really care if people actually read the contents" so I usually just close it. Unless I know from a reliable source that the contents is actually something very very interesting, that I would want to read then I'll take the extra effort and try to read it.
