[nitwit005]

If you turn down the brightness, the background and text will become more similar in color, as both will move closer to black.

A dark background reduces total brightness without that effect.

[topspin]

Exactly. Contrast or, more precisely, the dynamic range we desire, is ruined by dimming displays enough not to cause pain and fatigue.

It's a dumb solution, and we may all safely ignore it, knowing it won't occur. "Dark mode" haters will just have to cope as dark becomes the default.