Why aren't there electric cars with solar cells built on top of them?
First, let's have a look at current electric cars made for everyday's  use. I've choosen the Tesla Roaster, Model S and X and BMW i3:

                       Roaster  S     X     i3
Capacity [kWh]           53    85    75    18.8
Range [km]              350   500   381     190
Mileage [km/kWh]        6.6   5.9   5.1    10.1


The BMW is trimmed to high efficiency, while the Teslas are larger and are a bit more racy. And keep in mind, this is official data. If this is as realistic as mileage data for petrol driven cars, then... well...

Sunlight at the equator gives us about 1000W/m², but the efficiency of available solar cells is somewhere below 30%. So you get just about 300W/m². 
Next, the sunlight has to hit the solar cell perpendicularly to get the max. power. But if you place the cells flat on the floor / car, they generate just a fraction of their max power most of the day... In addition, weather reduces the power.

I've found a website which takes meteorological data into account to calculate the monthly energy output of solar cells. The site is german (you can switch to english, but I didn't find this calculator on the english version). It takes orientation (both 0 for flat on the floor), peak power in kWh  (0.3) and position (click on map), and I got this min/max values for winter/summer. Daily data calculated by division by 30:

             Monthly [kWh]  Daily [kWh]
California:      25 - 60    0.83 - 2.00
New York:        15 - 43    0.50 - 1.43
Germany:          8 - 48    0.26 - 1.60


The most economical BMW i3 makes between 2.6km (1.6 miles) in german winter to 20.2km (12.6miles) in californian summer per daily charge from 1m². I quess this car can not have more than 3m² of solar cells.

And keep in mind: This are average values. What if one or more months are unusual dark? And never park in a parking garage, in shadows of trees or buildings, ...

So, today, solar cells don't give enough energy per day for a car. But they would cost a lot, while a full charge from the wall outlet is quite cheap.