We don't truly have MAD with someone who has a small amount of nuclear weapons. Specifically one of the reasons the US dropped out of the ABM treaty was because

We don't truly have MAD with someone who has a small amount of nuclear weapons. Specifically one of the reasons the US dropped out of the ABM treaty was because they argued that they should be able to defend against countries with a small amount of nuclear weapons. It's also unlikely that everyone else launches their nukes during MAD, it's a doctrine that specifically applies to an attacker and a defender.