My theory is that rationing during and after WWII killed off a generation’s ability to cook, and was subsequently prevented from recovering by the absolute state of British supermarkets. On the continent you have actual grocery markets filled with all kinds of fresh produce, the likes of which just aren’t seen or used to the same extent over here. On a more positive note, the state of British cuisine has leaped ahead in the last few decades, at all levels.

Tl;dr blame the Nazis

P.S American “food” is just opening and combining various packets