What you're missing is that your eyes adjust the amount of light they let in and their sensitivity relative to ambient light, which can often result in a monitor having "inappropriate" brightness levels relative to that level. Nobody on the planet spends all day constantly adjusting their monitors' brightness and contrast levels, even assuming that it was possible to reach the appropriate levels in all ambient lighting conditions within the range of those controls (hint: it's not).
On the other hand, passive displays (books, e-ink) by their nature inherently match ambient lighting conditions, because they simply reflect ambient light. This is the equivalent of a monitor with a much wider range of contrast and brightness levels equipped with an automatic adaptive adjustment with sub-nanosecond response times, something that simply does not exist with present technology.
So yes, in theory a monitor could potentially match a passive display in terms of reducing eye strain and tiredness but in practice it is beyond the state of the art for now.
