A resistance heater also gives you warm air faster, and it works no matter how cold it is outside...heat pumps lose efficiency as it gets colder; in fact when very cold they can use MORE energy than a resistance heater, and may even stop working at all. Older heat pumps used to start having these problems not far below freezing, but I believe modern heat pumps are much better, and it's possible (?) that the lowest efficient temperatures are now low enough that nobody will hit these issues - I am not sure.
I am not arguing for resistance heaters; I think heat pumps are great, especially for me as I live in a fairly moderate climate. But there's tradeoffs to everything. Resistance heaters are small and cheap enough that I would think a premium automaker could have backup resistance heaters alongside a primary heat pump.