I would imagine this is down to the emission engineering ?
Most modern cars aim to get the engine up to temperature as quickly as possible so they can manage the emissions and during this phase it might be keeping the hot water within the engine loop? If the AirCon in a big BM is setup correctly it may supplement this to give you heat through an electric element?
Just boot it a bit more
My Ford engined Roadster has the engine quick warm up bypass and heater feed taken from the same cylinder head coolant rail, albeit former from the front and latter from the rear. Return for both arrive in the same mixing chamber feeding to circulating pump and prior to thermostat controlled radiator circuit. This places the two effectively in parallel and I imagine most modern cars work in a similar manner. There are no possibilities for a quick warm up to benefit emissions compromising heater output therefore.
Heater plumbing, core capacity and heated air ducting are another matter though.
