I hear this repeated often. But why? Whoever think it's not a good idea? There's a reason why contrast is usually maximized on readable paper. Text contrast is very important part of readability. I often see pretty much unusable sites, because someone thinks that it's so smart and cool to create sites, which content is clearly not worth of reading.
The other commenters here are right about the importance of text contrast, but there is a consensus in typography that pure white on pure black contributes to eye strain and decreased stamina when reading. This is especially true for people with astigmatism who experience a glowing effect around the letters known as halation.
While there is room here for both personal preference and accessibility needs for a true 21:1 contrast ratio, a better default would be to use a white on a dark gray background. Supporting a dark mode design is a lot harder than just inverting the colors, which this site appears to fall short on.
A slightly lower contrast may look nicer (and be more accurate to printed text) but often sites are excessive with it. It may not even be exactly the designer's fault since their monitor's gamma and contrast will probably be better than the average visitor.
Another related problem is that of font weight. Fonts too thin can also hamper readability but the weight on-screen will be variable with the user's font settings and how exactly it's set up with aliasing, shaping etc. To add to that some digital fonts are displayed at the wrong weight because they were designed with bleed in mind.
I'm using BenQ SW2700PT which was calibrated, and my room is also not bright at all. The pure white text on pure black background give me headache, but I don't have any issues on https://nim-lang.org/docs/manual.html dark mode which isn't pure black and white.