I feel like your initial question can be applied to a lot of things. For instance, Elher Danlos Syndrome has 13 variations depending on which gene is mutated. This include 11 known mutations and 2 catch-all categories for variations where we haven't yet found the mutation. Why are all these mutations under the same disease when they have such a range of symptoms? My guess is that it is because they were all lumped together before we had the language to talk about them as separate things or the technology to identify the causes. Going back and saying oh all these people don't have ASD or EDS or whatever but this specific thing that we have now identified might cause some disruption is research and awareness.

From what I know of cell biology (read: committed dilettante), the notion that one gene does X is rarely correct. The way to think about genes and the proteins they encode for is less “blue eyes” and more “metric socket wrench #5” - the protein is a tool, and lacking that tool means any operation that requires that tool isn’t going to function as normal, which is why you see such weird disparate effects from mutations and also multiple genetic pathways to the same pathology.

Also nature is absurdly cavalier about using whatever it’s got at hand, so it’s not so much “metric socket wrench #5” as “the pry-bar/hammer/thing we use to hold the hood open” that gets fucky when the genes go wrong.

You can invert that. Why do people with similar or the same genetic code (identical twins) have such variation in symptoms?

Ultimately, the symptoms are what people experience, and treatments often center on mitigating symptoms. Having an insight into the underlying genetics may or may not help anyone live with a disease, especially not in the near term.