The lunch buffet is close to a fixture at Indian restaurants across the United States. It persists because it fits both the cuisine's cooking methods and the constraints of American midday dining.
The food is unusually well suited to holding
Many Indian dishes are slow-cooked braises and gravies that have already been through long cooking, so an additional period in a warmer changes them relatively little.
Dals and legume preparations in particular hold their quality well, since they are uniform mixtures with no texture left to lose. The same is true of rice cooked to sit.
Cuisines built on last-minute searing or frying cannot do this, which is why the buffet format is far more common in Indian restaurants than in most other categories.
Lunch demands speed the kitchen cannot otherwise deliver
American office lunch runs on a short clock, and a diner with limited time will not order a dish that needs to be cooked to order.
A buffet moves the cooking entirely ahead of service, so the time between arriving and eating is a walk to the counter. Table turnover rises accordingly.
It removes the menu as a barrier
An unfamiliar menu asks a diner to commit money to a dish they cannot picture, which is a real deterrent for anyone new to the cuisine.
The buffet lets people see and taste before committing, which lowers the risk of a bad first experience and functions as sampling for the dinner menu.
Many restaurants treat it exactly that way, using lunch to build a customer who returns in the evening and orders from the full menu at a higher check.
The economics rest on predictability
A fixed price with unknown consumption sounds risky, but per-person intake across a lunch crowd is stable enough to plan around, and the food cost of legume and vegetable dishes is low.
Labor is the larger saving, since one buffet requires far fewer servers and no line cook working tickets during the rush.
What the format costs
Dishes that depend on being served immediately, including breads, fried items and anything crisp, either cannot appear or appear diminished.
The buffet has therefore shaped what many American diners think Indian food is, weighted toward the sauced and stewed end of a repertoire that is considerably broader than that.