I honestly don't even know if I can fathom how you could get permitted to build such a tank in a couple of weeks, but the answer to why that isn't happening _right now_ is probably because nobody wants to lay out a bunch of capital for a business that might do well contemporaneously but that has no future.
Either things settle back down to normal at some point, in which case you've made some revenue, but probably not recouped your investment. The Cushing facility managed to build 7 million bbl capacity in the early 90s for $60 million. I expect it would cost $80-100 million to build today. Current (inflated) storage costs are 50 cents per barrel. Let's say that rises a LOT and skyrockets to $2 - that's a gross of $14 million per month. Assuming $0 in overhead, you'd need for this contango to last for another 7 months to break even. If we assume something like a 30% margin, then you'd need it to last around two years.
Alternately, things don't return to normal. The market becomes depressed enough that airliners barely fly, and the global world consumption falls. The oil you're storing becomes cheaper to the point that storing it costs more than buying it, and there's nobody to sell it to. Your customers default, and your investment quickly becomes the thing that bankrupts you.