Because we’re not fixing the problems, only treating the symptoms. Those genetic defects becoming more widespread means that the extreme cases become more popular as well, which will, while they are still manageable, be inherited to the next generation. Eventually defects will reach a point where they are just bad enough to be treatable for most people but still crippling to a good portion. Take bad eyesight for example, if no gene manipulation takes place, eventually most people will need glasses or surgery to fix the inconvenience. Unfortunately at hat point some people will have eyesight so bad that neither glasses nor surgery can fix the, at that level, crippling defect. The issue here is that an average increase in mild incenvience means also an increase in extreme cases of the inconveniences. And I don’t think it’s in the best interest of anyone to work towards more crippled people. Not because cripples are somehow subhuman but because being crippled means a life filled with needless suffering. Being able to bypass deaths due to these minor issues saves lifes, unfortunately it can also mean that more people will die/suffer far worse effects than if we never had invented remedies for these problems.

I think long term, despite ethical issues, meddling with our DNA to get rid of known defects is better than letting nature run its course.