It is easy to be overshadowed by the impossible strength of constructs, monsters, and demons. To see Humans as "weak" and "lesser".

But It is only through the consumption of Human Flesh that demons can sustain their energy. When kept in captivity and purged of their energy reserves it becomes immediately clear that they do not have the innate Magica gathering ability that Humans have. 

Constructs as well seem to run on a "battery" of a kind, that must be recharged. Yes, it is true that they can exist for a long time this way, but they often go dormant for hundreds of years to sustain themselves.

Monsters are the closest to Humans in their ability to gather magica, but they do so through the consumption of slightly magically charged vegetation and other creatures.

In reality Humans have been given a gift that is so plentiful we totally fail to appreciate it.

-Theories on modern biology, by the Head Priest