Because I think randomness is far more fair, and humans deciding it on their tastes seem unethical to me. If you're randomly made, you just are that way. If you're tailor made by your mom and dad, it might make you resent them because you don't like their choices, or they in turn might think they can control you far more in other areas of life if they can control you there. It could easily become a cultural thing.

And I also feel like, if there is a future with that in it, then eventually kids who were born naturally like we all are will be treated by others and possibly their parents as kids who are unplanned 'accidents' are today, and in extreme cases, might be called unwanted, to continue with the cultural aspect of it. 

But then again, this could all be false and it is my personal opinion on it being unethical. We wouldn't know unless it happened for the other things as well.