It's kind of interesting that we started computing with dark mode and when we jumped to windowing systems, for some reason that coincided with move to white on black. Maybe it was more familiar for office workers used to paper.
There's no absolute reason why the evolution happened that way that I can say. It was all more or less arbitrary design decisions.
This old Sparcstation I managed to get for free years ago would display everything in "light mode" by default. If you would boot Linux or NetBSD to the console it would be a white background with black text.
Image: http://www.computinghistory.org.uk/userdata/images/large/21/...
I don't know how common this was, or if it's something that was unique to these sparcstations.
I think it's because computers like the Mac were geared around desktop publishing.
As long as designers understand and support both light and dark, it's good to have the option.
For me, any time I pop open HN or Google Drive, my eyes have to take a second to adjust to the glaring brightness compared to everything else on the screen.