Few cuisines ask diners to specify an intensity at the point of ordering, yet it is standard at Indian restaurants in the US. The practice follows directly from how the kitchen is organized.
Shared bases cannot carry a fixed heat level
Restaurant gravies are prepared in quantity in the morning and used across many dishes, so heat cannot be committed while the base is being made.
If it were, every dish built from that base would arrive at the same intensity, and the restaurant would lose whichever half of its diners disagreed with the choice.
Leaving the base moderate and adjusting per order is the only arrangement that serves both, which is why the question gets asked at all.
Late-added heat behaves differently
Chili cooked into a dish from the beginning integrates, mellows and spreads through the fat. Chili added at the finish sits sharper and more forward.
So a plate ordered hot is not the mild version plus more; it is a differently balanced dish where the heat arrives separately from everything else.
Cooks who have time will bloom the additional chili in hot fat before it goes in, which brings it closer to integrated, though not all the way.
The scale is local, not standard
Medium in one restaurant is hot in another, because each kitchen calibrates against its own regular diners rather than any external reference.
Chilies themselves vary too. Heat is not uniform between varieties, or even between individual pods of the same variety.
A diner therefore learns a restaurant's scale rather than a general one, which is why ordering the same level elsewhere produces surprises.
Heat is only one of several dimensions
Sourness, pungency from ginger and garlic, and the aroma of whole spices all contribute to what people describe as spicy, and none of them are chili heat.
A dish can be intensely spiced and barely hot, which is common in several Indian regional traditions and confounds a mild-medium-hot scale entirely.
Asking for more whole spice or more sourness is a more useful request than asking for more heat, though it requires a kitchen willing to have that conversation.
Adjusting at home instead
Cooking at home removes the negotiation, because heat can be built in at the base and the whole household eats from one balance.
American supermarkets stock serrano and jalapeno peppers that work well as green chilies, and cayenne stands in reasonably for standard Indian red chili powder.
Kashmiri chili powder, sold at Indian groceries, provides color with modest heat, and it is the ingredient that explains why restaurant dishes look fiery without tasting it.