It adds up to 40 miles per day, not 5. 5 would be a cloudy day.
I used to be anti-solar-panels-on-cars back when solar panels were expensive, and ones of reasonable efficiency were even more expensive - the argument was, "put it on your roof where it belongs". But that's just not the case any more. Adding solar is just not that much of a cost to the car. It adds some complexities, but mainly in the design / early manufacturing phase.
Also:
The average American drives 37 miles per day.
1) So if you're in a sunny climate, it covers all of said average-driver's driving. Otherwise, said average-driver has to plug in occasionally, but not nearly as often.
2) Most people drive less than the average (the average is skewed by a long-tail - small numbers of people who drive very far every year). What you actually should be meaning is the median US driver; the median drives 23 miles per day. Most Europeans, even less.
3) Even for said "average american", their daily average is skewed by long drives (e.g. road trips and similar). Wheren of course you're plugging in, you'd be plugging in even if the car was adding 80 miles a day. But when not on road trips, their daily average is lower.
4) Surely you can see the appeal of the tangential benefits, such as being unstrandable - where even if you run out in the middle of the desert 20 miles from the nearest town, you're still going to get there, just delayed (remember that EV ranges, if you drive very slowly, increase like 2x, so 40 miles a day becomes 80, so a 20 mile shortfall is only a ~4h delay on a sunny day).
5) Nobody is saying, "One car for everybody". Of course appeal varies by person and by location. Here in Iceland for example we have three problems. One, very little sun at all for a good chunk of the year. Two, even in the summer, when the days are long, the sun is mainly low and circles around you. Solar power just kinda sucks here in general. And three, the three-wheel config would mean that the centre wheel wouldn't align with tracks in the snow from other cars (although there is a slight advantage, in that it also wouldn't align with road ruts from studded tyres, which often fill with water in the rain and become hazardous).
But somewhere in the southern US, it's a great option.