Crude oil is sketchy stuff. All sorts of VOCs, etc evaporate from it. Some valuable, some, flammable, some will merely kill you. Keeping it confined makes all that more manageable.

TFA lays it all out, oil out in the open is exposed to attack/lightning strikes/etc.

Are you advocating for long term crude oil storage in open air oil lakes?

If the sides of the pond collapse (which happens a lot with mine tailings), then you have something resembling a oil pipeline rupture (which also happens a lot, but some states make it illegal to get video footage of it) which is a rather toxic situation needing soil to be bulldozed and either buried or burned.

Sun, rain, hurricanes, wind, debris is not friendly to long term oil storage.