Unfortunately, real life isn't that simple. I would love to find the genes linked to dementia, which people in my family are genetically predisposed to and I consider a fate worse than death, and remove them. However, not only will that not necessarily end dementia (which is more than just genetic), but such knowledge could later be used for eugenics, which is a very bad idea. Look at what's happened with dogs like golden retrievers-- they were overbred for reliably good traits, and while that has resulted in awesome dogs, its also resulted in recessive gene related problems such as a propensity for hip problems.
Plus, you don't know how bad its going to be. Some people with Down's syndrome are able to live happy lives, and there are different levels of being affected by it that you don't get to find out about until the person gets past the baby stage of their life. And there are pills and diet plans these days to manage cystic fibrosis, allowing for CF patients to live long lives. There are actually benefits to having sickle cell disease (namely, resistance or immunity to malaria). Heck, many of the screening tests are outright wrong-- they start with a 90% chance of accurately predicting chromosomal problems and decline from there with each passing week. Diagnostic tests are almost 100% reliable, but they carry a risk of causing spontaneous abortion. Spontaneous abortion is a rare event, but its one that a lot of parents with a wanted pregnancy don't even want to risk, especially those who have decided that they will love their children no matter what. Are we to imply that those who don't test as free of genetic abnormalities are doomed from the start? That they can never be happy? Have no right to live? No, of course not. At least not me.
Look, I don't want people to be forced to contend with these diseases either, but the solution is not gene editing. The solution is studying possible compounds for medication as well as any other potential treatments so that people are better able to manage and treat the diseases. Or alternatively, for parents who cannot afford to take care of a disabled child, giving them the choice to abort so that nobody suffers from the parents' lack of resources for treatment. Those solutions are long, slow, and deny instant gratification, but long term, those are the best solutions.
