Except there are huge entry costs, and all that theoretical ROI disappears as soon as you enter the market, because the existing monopoly starts competing.
To make a profit, you need some advantage over the them, like efficiency, innovation or goodwill. If all you can do is exactly what they do but at market price, well... they can do that too.
If someone started building a whole bunch of housing in SOMA, do you believe that this ROI would disappear quickly, due to housing prices hitting rock bottom?
Do you really believe that would happen?