Note that kids can't choose for themselves, only their parents. How parents define "improvement" might not always be in line with what the kids would want for themselves either.
What if their parents are really proud of their red hair and then give their kid bright red hair that their kid wished they didn't have? It becomes weird when you can no longer point to luck for the reason you are a certain way, but instead have actual people that you can blame. And that becomes especially weird when you are talking about preferences that are not inherently good or bad, like height, hair or eye colour.