The main thing the US built to keep the Soviets from invading was the US Navy. The US Navy was and still is the largest and most powerful naval force in the world, and was easily capable of intercepting any Soviet invasion force before it got close. The Soviets had a large naval bomber force, and an excellent submarine force, but these were primarily defensive weapons, designed to raid convoys and sink American ships attempting to attack Russia. Soviet surface ships were generally heavily armed and fairly capable, but they lacked carriers and would have been unable to approach the US mainland if even a small portion of the US Air Force or Navy was able to engage. Any invasion of the US was simply so unlikely that wiring bridges and building forts would have been of very limited utility. Even if the US was somehow successfully invaded, the Soviets would be completely unable to logistically sustain land forces in the US, and small US land forces could probably contain the Soviets long enough for them to run out of fuel and ammunition.

Even the airborne operations like the one seen in Red Dawn are pretty much militarily impossible. Getting transports through US Air Force coverage over such a long range would have been impossible, and lightly armed airborne divisions would have a rough time against US army forces.

The most likely way for the Soviets to invade the US would likely involve one way teams of special operations troops, like Spetsnaz being landed by subs to sabotage whatever they could, but even that would be a high risk, low reward option for the Soviets.

All in all, the combination of the US navy and distance made a Soviet attack on the US unlikely to the point of impossibility, and therefore building defenses was just not done.