In my home heating I have both (variable speed heat pump and variable speed air handler saves power bigly, but heat strips come on the few times it gets too cold even here in Alabama for the heat pump). The same with my hybrid water heater (heat pump built into the top to efficiently, but slowly, re-heat the water after a shower, but I can switch it to using electric elements like a normal electric water heater if I have company staying over and need to heat my water faster).
Most likely because of cost. Car companies will pull a nickel out of a vehicle if they can - anything times a million the cost matters. One might argue a Tesla is an expensive vehicle so it should include them. One might have similar arguments on 20 different aspects of the vehicle - costs could get out of control.
I’m not saying they shouldn’t or that I’m right - just my experience in the automotive field.