The problem is the cell modem in the car, which is hard to replace. Cars last a lot longer than phones do. When whatever network that the car uses shuts down, then you can’t remote start your car. That’s a marginal cost that the car company has to pay for.
It’s possible, but it costs money to design the hardware so it’s accessible, it has to use a connector which has to be robust against vibrations (is m.2 robust?), then there needs to be a standardized protocol to communicate with the card. Does the car computer need to know how to authenticate against the cell network or does the card? Is it industry standardized or specific to the manufacturer? All kinds of things need to be designed and car manufacturers have no reason to invest in they.