I think there are two key aspects to this:

a) The disease usually manifests itself when the person affected is in their 30’s or 40’s, so they have had time to have children, who then may carry the gene defect (50% chance to get the disease gene and 50% chance to get the functional allele)

b) Because there is 50% chance for a child of a person suffering from the disease to be healthy, the parents may want to try to have a healthy child, or, they do not know they have the disease and decide to have children like normal people do. The children then have a chance to become carriers (and in this case, future patients as well) depending on whether their parents had the disease and whether they had that 50% luck.

The disease would be eliminated or become drastically more rare at least in the following cases: if it was lethal at young age, if the people suffering from it or at risk would choose to not have children or if there was extensive genetic testing for embryos/fetuses at risk, and all those with the gene defect were aborted. I think we’re heading towards the latter option…