They use a resistor because it is the cheapest and most reliable (reliable as long term maintenance-free) method to produce "startup" heat. Granted, there is some strain onto the alternator and battery, but both are usually upgraded when equipped with an additional heater.
The mechanical resistance method would need some kind of friction coupling, subject to wear, tear and cost: Mechanical clutches/brakes do experience wear. Fluid resistance systems (similar as a calorimeter) need to have the fluid checked and are subject to seal ring wear. In all systems are some kind of clutch involved as you want to release the additional load when the engine is warmed up.
So in sum: It increases weight, risks, maintenance, complexity and costs.
The detriments are quite severe, especially since the following alternatives are already established:
- Some electrical heater, either powered by the battery or mains.
- A fuel powered, auxiliary heater. Available as air-heater or coolant-heater.
The mentioned auxiliary heater is also available as a pre-heater: This is the best (and most expensive) of them as the vehicle is already warm, the engine (in case of a coolant-heater) is also warm and the windows are defrosted when you enter the vehicle.