It depends on the total contrast on your screen. Unfortunately... On a high-contrast, high-brightness screen, white on black becomes glaringly bright and produ

It depends on the total contrast on your screen. Unfortunately... On a high-contrast, high-brightness screen, white on black becomes glaringly bright and produces halos, while a pure white background actually hurts your eyes. So you default to gray-on-gray because that's what's pleasant, while higher contrasts are used in scenarios where you want glare. (Games and photographs, mostly.) On a low-contrast screen, which describes a lot of cheaper hardware, the exact opposite is true. The real issue is it's all relative to what your display hardware can do, instead of using absolutes. HDR modes fix that, but HDR is still rare.