Because it allows them to hide one of the few characteristics that they can't shapeshift. As long as they don't open their mouth, they can mostly blend in.
Before eugenics, we had the natural order the world creating a harsh distillation of the human spirit. This gave rise to advances in every facet of spritual, scientific, and philosophic understanding of the human condition as never rivaled by other processes.
As we approached this understanding and followed its logical conclusion, we determined that this process could by synthetically accelerated beyond the natural pace, and with the accuracy of a scalpal, humans could surgically improve our species, compared to natures cudgel of random adaptations. We called this Eugenics. The careful and intentional selection of the best of all humans is how it is characterized today. This is a false argument in its premise. The intention of Eugenics (and possibly its shortcoming) is that it did not seek to have only the strongest humans pro-create. It only sought to limit the procreation of the lowest and most primal examples of humanity that were crude corruptions of human mind and spirit, the damaged and dangerous wolves that lay among the sheep.
Then World War II happened. The end of the German government at the hands of the Allied Powers would cast Eugenics as a corruption of its intent as described above. The misunderstanding of the Eugenics movement, as it existed, was an intentional creation of the "winning" side to villify their opponents.
The subsequent reaction to this casting has created a society where the "best of us" is encouraged to allow "the least of us" the space to grow and exist regardless of its effect on humanity. This was a societal overcorrection, bred by the propoganda of the industrial war machine that placed the USA and most other world nations into the most disastorous wars in human history. In turn, this led to the most unintelligent and incapable of society being placed under an umbrella of protection that allowed their rapid multiplication. To suggest that such a practice would be detrimental to the advancements of humans was now slogged and enshrined in accusations of racism and evil with nothing more than what amounts to the winner casting his opponent in less than favorable light simply to justify his own aggressions.
