It's a deceptively simple math problem. You only have so much surface area to cover in solar panels and solar panels only produce so much energy per square meter. You can fit maybe at most a kilowatt of solar panels onto a vehicle if we're being generous, and those panels aren't going to have optimum placement for power generation, and solar panels don't generate their peak output for more than about an hour per day, so the rest of the time will be significantly less than that. Let's be generous and say 3 kilowatt hours is being generated by those panels in an 8 hour workday of being parked in the sun.

3 kilowatt hours is a lot of energy, but let's not forget that about 20% will be lost to battery and circuit inefficiency, so 2.6 kilowatts is ultimately useable. But here's where it really falls apart: electric cars are extremely energy-hungry because cars in general are super inefficient. You're literally moving a ton of plastic glass and steel around, accelerating and decelerating repeatedly, all to transport on average 1.2 humans. Electric cars consume about 150 watt hours per kilometer in their regulatory tests. Some are more efficient, some are less but no matter what it's a ton of energy. That generous estimate of 2.6kwh will give you a whole 17 kilometers of range.

In short, the solar panels on electric vehicles are for show. They exist to trick buyers into believing they're more efficient than they are, and probably they'll help ensure a parked car doesn't lose too much range just from sustaining itself while parked. Also private cars are an extremely inefficient method of transportation