saves your battery
Maybe, maybe not. For OLED screens, where the pixels themselves generate the brightness, then an overall darker image will save power. For LCD screens with backlights it's the opposite: the backlight is always on and the lowest power state of an individual pixel is to let the light pass through unmodified - the part that costs power is turning the pixel 9n so that it blocks the light to make a black dot. So, your statement isn't true for all (or even most) devices.
Next: I find bright text on a black background to be hardest and most jarring to my vision. Humans have been reading black text on a light medium for millennia; it is natural. Light mode, for me, is easier to read and least tedious for my eyeballs.
I also just think that a light mode look is more polished looking...cleaner.