I think it's pretty rational.
In this universe, humans are literally god's chosen people. Before they got the gift, they were utterly owned by everyone to the point where the god of this world gave them a freebie, then they had to luck into a massive plague to surpass the other races.
Of course, the human nation misuse it and their god decides to level the playing field (not by giving every race healing magic; by giving knowledge of medicine through the main character).
Consider all of the other races shown thus far. You have ancient dragons that could annihilate armies. Dwarven crafts demolish anything a human could make. Academics? Try a race that lives for hundreds of years. Textiles? A single spider girl can weave out clothing better.
Healing magic is the only thing the human nation has. You can see how much they feel their worth is in chapter 5. The fact that they can't heal the big dragon lady doesn't fill them with the attitude of "unfortunately, there is nothing we can do" or "oh well, she was only a dragon and not a human." They immediately shit their pants, think that the dragons will wage a war over it, and they will be executed.
And it's not like all humans are ha-ha evil. It's shown that some are misguided and some do not agree with the philosophy of the human nation.
