It's really their dumb mistake. Back in '57 they probably didn't even think there would BE an America in 2007 much less anyone alive and around to open the vault.
Oh, I think the people of 1957 were more optimistic than that. More likely they figured that after the future Tulsans had lifted the car from its vault with hovercraft, they would simply zap it with a Restore-O-Matic ray that would turn any rust to gleaming steel. And afterward the workmen would enjoy a fine lunch (delivered via pneumatic tube) of white bread and Veg-O-Pills, followed by a couple of Luckies.