It generally costs too much for any sort of additional utility it creates.

A quick calculation. An average EV would have about 50 Kwh battery, that is 50,000 Watt hours of battery. The average solar panel produces 200 Watts/m^2 and even optimistically, there wouldn't be more than 4 sq meters of space on a typical car for solar panels. So the panel would charge at 800 Watt hours per hour of charging. A typical solar panel parked in most places on a sunny day achieves about 4-5 hrs equivalent of "full sun". So this means around 4,000 Watt hours if the car is parked an entire day, charging. This is less than 10% of the battery capacity.

In practice (not so sunny days, parked in garage, temperature, shade etc) a solar panel mounted on car would be lucky to achieve even 25% of this. So this amounts to 2.5% charge on a typical car battery.

This is simply not worth the cost of the panels and electronics since just the additional weight of the panels and electronics probably increases the electrical usage by 1-2% (the car now has to carry the panels etc). So the net result is minimal and would just never justify the cost.