Mainly, I'm just sincerely asking the question of the title. I'm interested in either explanations or justificactions, and I don't assume that there is no justification. I sort of suspect that there is little reason for philosophers not to collaborate more broadly, but I haven't looked into this much and would be happy to be corrected.
If not, why not? My sense is that there is just as much justification for collaboration with many other fields as there is for collaboration with psychology. Even in philosophy of mind, there are plenty of going theories of mind that would make linguistics, sociology, and biology directly relevant. But what about ethics and political philosophy? There seems to be lots of interest in working with psychologists, but why not sociologists or anthropologists? I'm particularly interested in non-government-centered political philosophy - that is, political philosophy that begins more with social movements, asking questions about the legitimate demands of liberatory practices rather than starting with governments and asking about their legitimacy. So I would think we might be able to learn a good bit from well designed sociological research.
Or in the more natural sciences, is there a reason why collaborative experiments are useful in philosophy of mind but not in philosophy of language, or metaphysics, or philosophy of biology?
Maybe so - and if y'all know of good reasons, I'd be interested. If not, maybe there's some dissertations out there for enterprising young folks.