British Indian restaurant food is often described as watered-down Indian cooking. It is more accurately a separate cuisine, with its own methods, developed under commercial pressures that did not exist in India.
The workforce shaped the menu
A large share of Britain's Indian restaurants were opened by migrants from a small number of regions, most notably Sylhet in what is now Bangladesh. The founding cooks were not a cross-section of the subcontinent.
Their menus nonetheless advertised generically Indian food, because that was the category British customers recognized. What appeared on the plate reflected the kitchens' own repertoire and their reading of local taste.
Recipes moved between restaurants as staff moved, which standardized the menu across the country far more than any regional Indian cuisine is standardized within India.
Service speed drove a technical innovation
Restaurants needed to send many different dishes to a table within minutes of ordering, which is impossible if each gravy is built from scratch after the ticket arrives.
The answer was to prepare a large quantity of a neutral cooked base in advance and finish individual dishes from it to order. That division between preparation and finishing is the defining technique of the style.
No Indian home kitchen works that way, because no home kitchen faces the problem. The method is a restaurant engineering solution rather than a culinary tradition.
Dish names became a heat and richness scale
Names on a British Indian menu often function as positions on a spectrum rather than as descriptions of regional dishes. Customers learned to read them as levels of chile heat and creaminess.
Several of the most famous names on those menus have no established counterpart in India at all, having been created in Britain to fill a gap in that scale.
The style traveled onward
British-format Indian restaurants opened across the English-speaking world, and some early American Indian restaurants inherited the same menu structure and the same base-gravy method.
That is why an American diner can find near-identical menus in cities with no connection to each other. The format traveled as a package, complete with its dish list and its kitchen workflow.
It is now old enough to be traditional
The style has been cooked and eaten for generations, has its own established techniques, and produces food that is recognizably itself rather than an approximation of anything.
Treating it as a cuisine in its own right, related to Indian cooking but not a substitute for it, is more accurate than judging it against home cooking it was never trying to reproduce.