Because for a lot of people, germline editing is the genie in the bottle. Specifically once it becomes accepted for morally justifiable things, like curing hor

Because for a lot of people, germline editing is the genie in the bottle. Specifically once it becomes accepted for morally justifiable things, like curing horrible genetic diseases, it will quickly start being used to alter non life threatening things. And we will quickly find ourselves in a world where parents are able to alter the future children's height, intelligence, muscle mass, sex, race, sexual preference (These are purposefully extreme examples that are likely many decades away from being possible). But in terms of why there is moral outrage, those are the sorts of things people are concerned about. I would say that from the reading I have done, the concern that it might be done improperly and accidentally cause negative mutations harming the child is more of a practical one (we don't understand the genetic processes well enough to perform the editing safely yet), rather than the moral one outlined above.