Why aren't double decker bridges more common?

So BTR can have 2 gridlock bridges?

People who build bridges aren't interested in saving money nor are the people to whom they pay kickbacks. If it saved money, you can be confident organized crime wasnt heavily involved.

The I-71 & I-75 Brent Spence Bridge spans the Ohio River, connecting northern Kentucky and Cincinnati

Never though about it but it’s a great idea.

The incompetents in BR couldn't build a double decker sandwich, much less a bridge.

when you have two spans and one closes for maintenance and or repair you have options with one span not so much.

As others have noted, soil composition must be such that the heavier loads can be supported. This factor alone decreases the number of available sites.

Helicopter medievacs can be handled by simply ambulancing the patients to a point where the airlift can be managed.

But another point that makes these bridges more difficult is that the trusswork must be stiffened so that vibrations on one level do not transmit to the other level.

You can take my word for all of the above, I studied at the Wikipedia School of Engineering.

If you put the BR how the hell are we supposed to rubber neck?

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It seems like this would save a ton of money both practically and real-estate wise.

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Not sure what you mean by saving money "practically". You still have to support 2 roadways, so, it is basically the substructure (or support structure) that you think would be saved. That saving may not be so great as it will basically have to be built twice as strong. Throw in the complexity factor and the money saved is going to be minimal, if at all. As far as real estate savings, the area consumed over the object it is crossing is usually cheap or "free" anyway. For example, a bridge over the Miss. River is not going to require real estate acquisition over the water. The real estate costs are on land, where the roadways are, not the bridge.