Sure, the air-gapped system gets infected, BUT the problem is that people connect via USB drive to an infected media and then use that same drive in a “clean” machine! So, for example, maybe I was not sleeping around, but my husband or wife was, and I became infected with an STD through them!
Basically, there should be NO thumb-drive action on an air-gapped system. Because then the system is no longer “gapped.” Unless you use special thumb-drives that have not and will never be in an Internet-connected machine. But even that should be rare and highly supervised.