There is a little bit more to an oil tank than just steel walls. You need proper foundations, fire prevention systems, disaster plans, probably get hooked up to the pipeline network, etc etc etc. Even if it were quick, the amount of oil being pumped up is massive and you need more than a few tanks to store it all. Finally, tanks are long term infrastructure and it might not be profitable to build out storage too much just in the hope of catching once-in-a-lifetime negative pricing events.