Why is it that you make stock (and/or gravy) from a roast chicken or beef or fish, but you don't do the same for pork?
Ham bone soup and red eye gravy are some common preparations (in the southern US, at least) made from pork leftovers. We'll also make sawmill (white) gravy with cooked sausage. It's delicious over biscuits.
If I boil a ham or gammon hock, I'll often use the stock from that to make pea and ham soup. Its a) delicious b) Cheap and really easy, basically involving adding a bag or 2 of frozen peas and some of the meat from the ham.
The flavor I think is why you usually don't see many other stocks.
Beef makes excellent general use stock, but fatty meat such as lamb, pork and oily poultry like duck and goose have too distinctive a flavour.
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I can attest to the lamb stock. I made a sauce from a lamb leg bone and the flavor was dominated by strong lamb taste.
I think it is because it is so greasy. But they do use pork as a base for things like baked beans.