An American diner who has eaten Indian food many times has often eaten a narrow slice of it repeatedly. The reduction of a subcontinent's cooking to a short familiar list has traceable causes.
Migration decided which kitchens traveled
Restaurant cooking abroad grew from the regions that sent the most people at the moment restaurants were being opened, not from an even sample of the country.
Cooks brought the repertoire they knew, and the earliest successful menus set the template that later arrivals had to compete against. Following the established form was safer than explaining a new one.
The result is that whole regional traditions, with different staples, fats and techniques, simply never entered the export version.
A shared gravy base makes a menu affordable
Restaurant kitchens build many dishes from a small number of prepared bases, which lets one line cook produce a long menu quickly during service.
Dishes that need their own distinct foundation are expensive to carry, so they drop off menus even where a cook knows them well. Economics filters the list more aggressively than preference does.
Diners then read the resulting similarity as the cuisine's character, when it is really a kitchen workflow showing through.
Naming conventions hide the variation
Menus abroad label dishes by their sauce or their protein, which suits ordering but erases the regional identity that would otherwise distinguish two similar-looking plates.
A dish cooked three ways in three states arrives under one name, and the differences that matter to a home cook become invisible to the person ordering.
Home cooking never had this problem, because at home a dish carries its family and place with it rather than a menu category.
What an American grocery run reveals
Walk an Indian grocery in the US and the shelves contradict the restaurant picture immediately: several distinct rice varieties, a dozen lentils, regional pickles, and fats from mustard oil to coconut oil.
Mainstream supermarkets stock a much shorter list, usually curry powder, garam masala, basmati and canned coconut milk. That shelf is roughly the restaurant menu translated into ingredients.
Cooking outside the familiar list generally means one trip to a South Asian store or an online order, which is the practical reason home cooks stay inside it.
The gap is closing from the home side first
Regional cooking spreads abroad through households and small operators before it reaches full-service restaurants, since a family kitchen carries none of the menu risk.
Dishes that need a griddle, a steamer or a fermented batter appear at home long before they appear on a printed menu, and often never make that jump at all.
Anyone wanting the wider picture gets there faster by cooking from a region than by ordering from one, which is an unusual thing to be able to say about a cuisine.