It doesn't because e-ink is a non-light-emitting technology so has no concept of brightness. Ambient light is what lights it up usually (unless one turns an entirely optional backlight on to read in the dark). So you don't need to adjusts as there's nothing to adjust, it's literally ambient light bouncing back, so is the same brightness by virtue of being the same light source.
Conversely, typical displays (TN, IPS, OLED, CRT even...) being a light emitting technology, one needs to turn backlight up as ambient light increases as both compete. (That is, unless it's a transflective LCD, but even then it's quite dim in ambient light).
> there's nothing inherently less tiring about it
e-ink really is more like paper, while typical displays are like turning a flashlight straight towards your eyes.