It is very hard to model the effects of magic on those kinds of things. This is a heavily unexplored area of fantasy, in my opinion.
Not
just magic, but monsters too. The simple fact that humans are not at the top of the foodchain would have serious effect on evolutionary biology and behavior.
How would a society respond when there are things that can literally take your appearance? Human cultures are xenophobic - when a stranger (let alone your neighbor) can potentially do
anything, paranoia would go into the atmosphere. The existence of undead would also have some serious changes for society. How about races? Look how human beings have treated one another throughout history - if there was a race that was certifiably
not human, created by a non-Human deity... how could any peace be
possible? Humans would have wiped them out.
It would also reverse many of the tropes we associate. A castle or keep with a fortified wall and courtyard
would not work because so many monsters (and spellcasters) can just fly right over the defenses and plop down in the courtyard.
But no one takes into consideration the sheer
amount of
food that monsters would need in order to exist, let alone in large numbers.
It's just easier to handwave all this stuff because most people 1) want their tropes, and 2) aren't economists/sociologists/biologists, 3) aren't really interested in it, and 4) the end result of a world that takes those considerations in may very well
not be fun, even if it's realistic.