Well, it's all good to say that a genetic component must be there, but no one really knows for sure. We all believe there is a genetic component, but none has ever been identified; it just seems to be what makes the most sense given the evidence. But we don't know if it's really there or how it actually works.
You're right that we don't know what gene or what set of genes govern the formation of the hip socket and the ball joint, but if you see clear heritability of a trait, it's has to be genetic or epigenetic. You don't have to specifically isolate the gene or genes in order to have conclusive evidence that something is heritable.
It's also noteworthy that dogs raised on better quality food seem to have a lower rate of HD, though that too is hard to say for sure because there have been no studies done on that.
If there's been no study, how do you arrive at the conclusion that there seems to be a relationship between food and the rate of HD? There's some evidence of the relationship between the expression of HD and weight, and there's been some evidence that calcium, phosphorus, and excess calories during growth spurts can affect bone formation, that's not really about "quality food," which you seem to define in terms of common kibbles vs. boutique kibbles and alternative diets.
There was low incidence of HD before 1950 because people shot lame dogs instead of getting x-rays.
I think Julie is probably right about a combination of some sort of genetic predisposition combined with some sort of environmental trigger being the most likely cause. But we still don't really know. What's taken as conventional wisdom is just that; it's not science.
But it is science. OFA has demonstrated, very conclusively, that HD and ED are heritable by examining the rates of the disease based on the status of the parents. Breeding clear dogs cuts the risk by about half. What other explanation could there be aside from heritability?
Sorry to hijack, OP, but it's actually pretty relevant. The heritability of HD (and ED and SAS and PU) will highly affect your search for a litter, because you will only want to go with litters that have a rock-solid clearance history on hips, elbows, hearts, and eyes. Even if the breeders get positive comments on GRF and elsewhere, you still need to examine individual litters for their clearance history before you commit to a litter. The most reputable breeders will invite this level of scrutiny and recognize it as a quality of someone who's done their research.