That's probably part of it, but also the "mutually assured destruction" is saving us from the mutually assured destruction. No one wants that horror in their own country, and no one believes their defenses are good enough to prevent it if missiles start flying.

This is probably realistic. Modern ICBM designs carry multiple independently targeting warheads, plus decoy warheads, but can't travel fast enough to reach a rival nation before they can launch their own missiles. If launched, every nation with such weapons has the capacity to completely overwhelm any anti-missile defenses, but not to prevent retaliation.

This situation becomes unbalanced when:

1. Someone develops a practical method for shooting down 10 actual warheads per missile while correctly identifying the decoys and either avoiding or shooting them down too.
2. Someone develops a faster delivery system and/or a reliable way to blind early warning systems (unlikely because you would need to blind multiple space and ground observation platforms at the same time).
3. Someone rises to power who doesn't care about the consequences to their own nation.

Personally I think the fear of dying horribly is still the largest factor. The US has probably invested the most in developing a practical ICBM interception system:

https://www.mda.mil/system/system.html

...and that pretty much just gets you to "we can probably prevent half the warheads from reaching their targets".