Silos are a means of protecting a retaliatory force from an aggressor's pre-emptive strike, and as such are more of a defensive, rather than offensive, weapon. The side that intends to strike first builds highly accurate silo-busters but makes no expensive effort to protect them since it intends to shoot first.

The defender doesn't need his missiles to be too accurate, since he is targeting cities and economic targets, not hardened military facilities that must be taken out early in the conflict. He spends his money on making his weapons very numerous, highly mobile, or encasing them in concrete, so they can better survive a first strike.

The problem is that it makes sense to mix and conceal your defensive and offensive capabilities to make your enemy's planning more difficult, which tends to make it more likely for him to misjudge your motives and intentions. This only increases the possibility, and severity, of an accidental conflict...

We're very fortunate our arms race with the Soviets didn't lead to disaster. We won't be that lucky a second time.

As the Fat Man says in "The Maltese Falcon"; "this requires careful judgement on both sides, since in the heat of action men can't be counted to act in their own best interest."