why shouldn’t it be the same on a screen?

ooh ooh I know!

https://en.wikipedia.org/wiki/RGB_color_model
https://en.wikipedia.org/wiki/CMYK_color_model

On paper, you use the subtractive colour model, so the light is reflected off the page, and the text is taking away from what's reflected.
On a screen, you use the additive colour model, so seeing brighter colours means more lights have to be shined directly into your eyes.

If you are finding white/bright text on dark backgrounds difficult to read, adjust your font size settings/thicknesses or check your eyesight out.