There is not enough solar energy provided by sun to make it affordable
Typical per capita electricity consumption in developed economies is 6–12 megawatt-hours (MWh) per person [4]. This may double to around 20 MWh per capita [5] to accommodate electrification of most energy functions.
The power and area of solar panels required to supply 20 MWh of electricity per capita per annum are 14 kilowatts (kW) and 70 m2, respectively, assuming an average capacity factor of 16% [7] and an array solar conversion efficiency of 20%.
For ten billion people, this amounts to 140 TW and 0.7 million km2, respectively. This can be compared with the global land surface area of 150 million km2 and the area devoted to agriculture of 50 million km2 [8].
The simple calculation above shows that the world has sufficient land area to provide energy from solar PV for ten billion affluent people.
TL; DR; full solar electrification with current technology for 10 billion affluent people is possible if we dedicated less than 2% of the real estate currently in use by global agriculture to electricity production