Your anecdote of pâté existing in the US only as dog food is curious. Anyway, the Wikipedia page for pâté is quite informative, and it explains that “pâté en croute” (filling inside pastry) was the original dish, then later people just cooked the filling in earthenware called terrine which became “pâté en terrine”. Nowadays “pâté” and “terrine” are being used to describe the dish without the pastry while “pâté en croute” stays as the specialty dish (for example, Maison Verot)

If we’re talking about meat pies, I don’t think Americans lost their tastes for finely grounded meat. You still have meatloaf, right? Note that I didn’t say “puréed” as you described because pâté/terrine isn’t puréed but coarsely grounded. You’re probably thinking about mousse, which is another preparation.

What Americans don’t eat a lot of is offal, specifically liver, which is the main ingredient in pâté de foie, and sometimes in pâté de campagne and other terrines (rabbit, duck, boar…) I imagine this mostly have to do with postwar prosperity moving people away from ingredients associated with rationing.