Because it’s almost always just done on a racial bias and not something voluntary targeted at eliminating genetic diseases.

Even then, a lot of that stuff has a racial lean. Things like Sickle cell may be a huge advantage in places with malaria, but a negative everywhere else.

There’s 100% some generic issues with no positive tradeoffs, but any system that would incentives them from not reproducing and passing on those genes would just be too open to corruption.

The risk/reward just isn’t worth it