Pretty much simplicity and size as well.
They're meant for near-field listening, so they don't need a lot of power because they're not meant to go night club loud.
I do agree. Different amp's give out different sounds.
You would need a very good amp that retains the flatness of the original audio.
Don't forget the really big passive monitors which obviously need an amp.
But if you're going that big, you would be spending a lot for high end amps and what not anyways.
So yes, based on pretty much monitors reproduce the "true" sound of the audio being played.
Otherwise, why would they be called monitors, or why would we even need speakers designed to reproduce audio as accurate to the source as possible?
It's easier to go straight for monitors designed for reproducing accurate audio than looking around finding good speakers and an amp that do the same.