Indian restaurants in the United States routinely ask diners to choose a heat level, a question most other cuisines do not put to a table. The practice reflects where in the cooking process heat is added.
Heat is a finishing variable
Kitchens working from prepared bases build most of a dish's flavor into that base in advance, then finish individual orders in a pan to order.
Chile can be added at that finishing stage as powder or as fresh chile, so it is genuinely adjustable per plate in a way that the underlying spice profile is not.
Very few other elements of the dish can move that late. Salt is already set in the base, and the aromatic blend cannot be altered without recooking.
Tolerance varies more than for other flavors
Capsaicin sensitivity differs substantially between people, and regular exposure reduces the response over time, so a single setting genuinely does not work for a mixed customer base.
Sweetness, saltiness and sourness vary far less between diners, which is why restaurants do not offer scales for those and do for heat.
The scale is local, not standard
Mild, medium and hot mean whatever a given kitchen decides, and there is no industry reference behind them. The same word produces very different plates at two restaurants.
Kitchens calibrate to their own customer base, so a restaurant serving mostly diaspora customers tends to run its scale hotter than one in a neighborhood where the cuisine is less familiar.
This is why asking what a restaurant's medium is like, rather than assuming it matches the last one, is the more reliable approach.
Heat is not the same as spicing
Chile heat is a single sensation delivered by one family of compounds, while the spicing of a dish is a complex aromatic profile built from many.
Ordering mild removes heat without removing spice, which is worth knowing, since diners often assume mild means bland and avoid it for that reason.
What the question tells you about the kitchen
A restaurant offering per-dish heat levels is almost certainly finishing from prepared bases, which is a legitimate and near-universal commercial method rather than a shortcut.
Kitchens that decline to adjust heat are usually cooking dishes where the chile is integral to the preparation and cannot be varied without changing the dish itself.