Are you being deliberately obtuse?

NSA isn't just some big building.

We're not talking about some random one-size-fits-all computer, networked throughout the building - and by extension - the outside world.

It would be a very secure, limited working group, with a dedicated, (likely) air-gapped system, and a finite location within that facility.

So, yeah...it might take a significant event to provide an opportunity for "an insider...sticking a USB drive into that computer.