Because even though panels are cheap at 150$ per 300W panel, the installation is 2–3 times that in most of the world, and 7 times that in the USA because of gov regulations, fees and paperwork.
The logical places to put solar pv panels are on roofs over building, parking lots and roads or road sides. These are all usually more expensive than open field installations, but not by much in most places except the USA.
In cities where the cultural norms dictate white and light colored buildings, even solar panels facing away from the sun will get 20% as much as optimum facing.
Note the cost of solar panels is now less than the retail cost of the same amount of low iron glass, solar panels are as cheap as tile and ceramic roofing, solar panels are around the same cost of aluminum siding.
This means it`s starting to make a lot of sense to build roofs out of solar panels, rather than add them to a regular roof. I would hate to see panels wasted in the dark.
So what should keep happening it so put solar panels in the optimum locations, at least till we have supplied enough to provide 60–80% of our total energy needs, then we can go for marginal locations, and by then the prices will be about half of now. 10–20 years.
The big advantage of large solar farms is they are easier to build and maintain. There are even robots that will fill an entire row at a time automatically.