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.

Basically what Neshura said, but here's some more context. Survivable is not the same thing as no longer a problem. I'm diabetic, it is 100% survivable, but it has also had a profound impact on my life. One of the things I have to consider when trying to decide if I have a child is if I want to risk passing this disease on to them. It's not 100% guaranteed my child would have diabetes because my wife doesn't, but it's a pretty good chance. If I had the option to get genetic editing done to guarantee my child wouldn't get diabetes I absolutely would do so because nobody should have to live with this.

That's just one disease, one single example. There are hundreds of genetic disorders that range from mild to severe impact on peoples health and life. Things that modern medicine can treat the symptoms of, and allow people to survive at least long enough to reproduce. Many of these diseases are still ultimately fatal, but not till much later in life. This is to say nothing of diseases like Parkinson's that are normally not symptomatic or fatal until later in life.

You're getting dangereously close to eugenics here, buddy.

I am aware that this is dangerous territory, but the need out weighs those risks, and if you notice I was very specific to point out that there is risk and we need to be very careful with how this tech is used. A perfect example of where we need to walk carefully is autism. Is that something that should be treated? Maybe, but probably not. Except in the most extreme cases it's not really something that's dibilitating, and there are also advantages to being autistic. That's one example where we should probably just let things be. But there are plenty of others where I'm sure everyone can agree stepping in is the right thing to do.

The biggest risk, the thing we must absolutely avoid, is the allure of a "designer baby". It's very tempting to give people the ability to do things like pick their childs eye color or tweak their height or do dozens of other things that ultimately are not medically necessary. That is the really dangerous path and the risks there absolutely outweigh the advantages.