[maskros]

Because astigmatism or other refractive vision errors.

Think about how astigmatism affects vision, then take into account the effect on pupil size based on incoming brightness, and how this affects the depth of field, and you may understand why.

[walteweiss]

Sorry, still don’t understand. Would appreciate ELI5 or other kind of explanation.

[arcanemachiner]

Some people have conditions that make dark mode text harder to read, not easier. Astigmatism is one of them. Also, myopia.

It can cause "halation" which is like a foggy glow around objects which are brighter than their relative surroundings. Like a streetlight at night, or white text on a black background.

I have this to some degree but I still prefer dark mode most of the time.