The cause of human malleability

Why are humans made into the other races rather than the other races simply having children? What if the non-human races cannot bear children? This may have been caused in an ancient war where grand magic was used to end the war, a curse on all non-humans so they could not sustain the war. After this, the malleability of humanity was discovered. A solution for the other races to not die out.

I believe that the other races can have children, but since they are native to this world, they can't acquire the power necessary to stop the evil (whatever we decide on it being). The humans are an abnormality in this world, something along those lines at least. I could be wrong though.

I like the Combination of the two, not to take the middle Ground.
Okay so My Take on it, The Other Races Can't breed but human changed members. Basically free from this so called "curse" or scar left by the old wars or something. That in itself is Causing a prejudice where these clay people seem to be able to have more power and the ability to breed where the original races can't. So the Main Races need and yet still dislike the Humans. Maybe Have it as an OverArcing Race quest to go out and heal or better the race

Its just an idea I'm throwing out there because of the restriction in players can only chose human.

Honestly it doesn't even have to be human, it could be a golem, or a goblin, we just want everyone to be the same race and be able to swap it once, to allow for top customization. In my opinion.