I think you don't get how genetics works.
Various biochemical pathways are conserved, and are there to react to different environmental conditions.
Disease is a moving target. As we suppress one disease, another takes its niche.
In most cases we only know the primary biochemical pathway, have sparse information on the secondary biochemical pathway which kicks in when the primary pathway is disrupted, and have little to no information on the evolutionarily conserved tertiary biochemical pathway we inherited from fish or dinosaurs.
Making "designer" kids leads us to less genetic diversity, and less adaptive capacity as a species to stressors which will occur.
That plus in most cases we end up with too many males and not enough females when we let people "choose".