The real question is “why do you enjoy starring into a torch all day?”
Before office applications, dark mode was the standard. Do you remember DOS? Linux, VMS, DEC terminals? All used dark mode, because that was the right choice, easiest on the eyes, and provided the best contrast.
Then somebody had the “bright” idea that paper is white with black letters, thus office applications have to follow the same color scheme, not to disturb the secretaries. It was also considered modern and fashionable at that time. However, with contemporary display technology, it was a disaster. The 50/60Hz flicker of CRT displays was well-known to cause eye fatigue. Making the entire screen flickering, instead of just the letters, made the issue hundreds of times worse. People were suffering because of the flicker. Medical articles were written. Doctors warned that prolonged work at the computer was harmful. Different filters and glasses were invented with no significant effect.
Coders continued to use dark mode. Cool or not, it works so much better.
With modern TFT displays, flicker is not a problem anymore. Contrast is good with almost any color scheme. However, you’re still staring into a torch. Why? It’s really inconvenient and bad for your eye.
Black themes are also better for OLED (and plasma) screens. Power consumption and display degradation are lower.