I don't think it's that those studies aren't incredibly revealing, useful pieces of research into pathologies whoknows. It's more that they form a part of a picture, and that they are subject to the same caveat of 'this is the best we know right now, and is inevitably as limited by current assumptions about the world and universe as any other piece of knowledge is right now'.
Let's run with your example of schizophrenia - this is not my area, so I'm deeply out of my comfort zone. I'm also thinking aloud here, so what I say is to be treated with a huge handful of salt!
Firstly, a point from within the current scientific way of thinking. Scientifically, we "know" (subject to above caveats) that a number of genes play a role in schizophrenia. Howeer, genetics as far as I understand it (which isn't that far - I have a biology degree but that's not exactly specialist) is moving away from the old "one gene, one protein, one function" hypothesis towards a much more complicated view of biochemical interactions, in which the difference between terms like 'environment' and 'genes' are no longer that clear. If a synaptic receptor in the brain of a small person (sensitive period unknown) is in a biochemical environment that includes lead, then that person is more like to get schizophrenia later on. But that lead is in the 'environment' of the gene at every level from the molecular right up to the fact that the foetus/baby is being brought up in inadequate housing where lead pipes haven't been replaced, which in turn is 'caused' by ideologically-laden ideas of the inequality of housing and prospect that we accept (scandalously, in my view) in our society. So the presence of the lead is biochemical, but it's also social and political. Similarly, the exposure to stress in young adulthood that can increase the risk of schizophrenia is simultaneously biochemical and social in nature: we find something stressful because of social and cultural factors. So at a biochemical level, even within our current scientific paradigm, our bodies are shaped by the social and economic, which seems to challenge any easy opposition between nature and nurture.
Secondly, a point that relativizes science. Nowadays, we describe this in terms of biochemical interactions, but formerly, in the nineteenth century, it would probably have been seen as a pattern of heredity, and in the eighteenth century as a condition of oversensitive nerves or overstrained sensibility (in the middle class, at least - though there were some nods towards inherited madness too). Three very different paradigms for disease - and they are not isolated from the surrounding culture but very much grow up out of the wider politics of the time. There's a relationship between eighteenth century philosophy about perception, the nervous openness of the body to the world, colonialism and broadening trade links; nineteenth century ideas of race, madness, disease and eugenics, heredity and imperialism abroad, with the rise of spatial control and surveillance at home; twentieth century ideas about biopolitics and the inevitability of neoliberalism (believe it or not, there is a huge literature on this, with which I am almost entirely unfamiliar 
). My point is that in no case is the scientific paradigm outside of the political (understood in this broad sense), because the very limits of what we are able to think are defined by the ideological.
I guess what I'm saying is that science, as a practice, can't escape its own historicity.