Everything digital in a car is often handled by the “entertainment” system. Like a glorified radio. Manufacturers like to keep that as separate system from the car, so it’s replaceable and upgradable and fail safe from the actual operation of the car.
Also, many car designs (of the cars on the road today) are 20 years old, when digital screens in cars had yet to prove reliability. Nobody wanted to risk having to replace screens just to show the speed. Some brands have had digital speedometers for ten years or so.
Anyway, digital speedometers also calculate the speed by magnets, so the GPS and speedometer might still show different speeds depending on the size of wheels just as badly as an analogue one. Again, it has to, because the operation of a car should not be dependent on a satellite system, f.i. in tunnels.
So in short: Digital speedometers are not more accurate and they’re introducing points of potential failure.