Because the w/m2 of a solar panel in optimal conditions (3-400W/m2) when applied to the area of a car is a rounding error compared to the energy consumption of the vehicle itself (5000-20000Wh), and those optimal numbers are degraded massively by the fact that the car can point any direction, and for aerodynamic and style reasons the panel will not have optimal angles even when pointed the right way. Best you could do is offset the 12V load to some degree.

There are cars with panels on the roof but just do the math. It’s a drop in the bucket, unless you live in the tropics and drive very little.