Because of the Nuclear Triad, precisely.

Take a look at the sea component. You have US missile submarines that are nuclear powered and on patrols for months at a time. Each one carries many very long range SLBMs and each missile is MIRVed, capable of delivering a handful of thermonuclear warheads each to separate targets. In the case of a more modern sub like the Ohio class they're capable of carrying well over 100 roughly half megaton warheads, each of which can hit a different target. That's a tremendous amount of destructive power. It means that even one solitary sub is a credible nuclear deterrent capable of effectively ending any country on Earth. Couple that with the fact that the US had a dominant blue water navy with carrier task groups across the oceans, plus a global sonar listening network, plus satellite based surveillance, plus a highly active attack submarine force and you end up with a very different situation for US vs. Soviet subs. Most US missile subs were able to operate unobserved in oceans that were largely unpatrolled by Soviet subs. Soviet missile subs, on the other hand, were essentially constantly monitored and consistently tailed by attack subs. During the first stages of an all out nuclear exchange between the US and the USSR the US attack subs would have had a very high chance of sinking essentially all of the Soviet missile subs.

Similarly, during the Cold War the US Strategic Air Command was constantly in a state of readiness and during periods of heightened geopolitical tensions would operate active patrols. SAC bombers had a fairly high chance of being able to hit Soviet targets within a matter of hours. This was substantially facilitated by all of the territories near the Soviet Union where SAC forces could be based.

Taken together all of this meant that each leg of the US "Nuclear Triad" was more or less an equivalently credible threat of nuclear annihilation, especially against the Soviets specifically, and each could stand alone without the others.

In contrast, neither the Soviet submarine or bomber forces could reach the level of credible survivability and reliability of delivery that the US forces could. That's a major reason why the Soviets sought other methods to increase survivability of their land based missile forces, and chose to go with mobile launchers.

The US did experiment with mobile launchers during the Cold War but there was never a sufficient justification to add another leg to the triad. Whereas for the Soviets mobile launchers rapidly became the most survivable leg of their deterrent system and coupled with their vast expanses of sparsely populated territory were a good fit for the resources and advantages available to them.

Additionally, silos are not at all easy to take out. It takes nearly a direct hit by a powerful nuke to knock out a silo, due to their reinforced concrete construction. And because silos are packed in bunches that makes it even harder. A reentry vehicle can't easily fly through the bomb blast of another warhead going off, so it needs to wait, giving time for any surviving silos in the field to launch.

Silos in the US were fairly outdated by the mid 80’s (most having been made in the early 60’s I believe.) Mobile launch systems were in the works, but never came to fruition before the end of the Cold War. As to why the US didn’t replace them sooner, I can only imagine it was because they didn’t feel the need. It’s unlikely all silo positions were known by the Russians, not to mention it’s a mighty big waste of a nuke on a single position nowhere near a population center.