Why do CMOS digital integrated circuits have high immunity to noise on the input signal or power supply?

Explication

A CMOS input switches at about half the supply voltage, so noise on the input or on the supply must be nearly half a supply rail before it can flip a gate — a far larger margin than families that switch near a fixed low voltage. A threshold at twice the supply could never be reached, and neither inherent bypassing nor limited bandwidth is the reason.