Pretty much this. It's actually pretty surprising to see the number of people who come up with solutions that assume Tesla has the right to do whatever they want for the super chargers. Tesla has to negotiate with the actual property owners at most of these locations, and not all of them will be amenable to Tesla taking up more room.
Plus, solar panels (and batteries to store power to offset demand) is an IMMENSE expense to cover for super chargers. As others have pointed out, super charging just one Tesla from zero to full would be several times what the average single family home uses in electricity in one day, and that's over the course of, ideally, less than one hour. The average single family home uses 911 kwh per month, which is a little over than 30 per day. That's only a little over half of the battery capacity of even the smallest Tesla battery (since the 60 has capacity reserved to keep it from bricking).
It might be doable at less popular SC locations, but then it would still be an expensive up front cost and wouldn't be defraying much expense. If Tesla tries to do it at the more popular SC locations in CA, the space requirements would be insane, not to mention the expense, and, perhaps most importantly, the time and resources and labor that would have to be redirected, all for a goal that would take years to pay off.