That would be fine if it was say a motel or a garage that was struck but it was one of the most secure buildings in the entire country, you're telling me they had nothing better than the 1 frame every 5 second camera they allowed to be shown of the impact ?
How many high quality/full frame rate cameras do you think the designers of the likes of the car parks around the pentagon thought they needed in order to catch an aircraft hitting it?
Even in high security places you place cameras to watch the expected threat/risk locations, and you specifcy them for the job.
So you don't whack a 4k 120fps camera to watch the entry to a car park, you don't put fast panning fully monitored cameras capable of looking in the air away from your building, you don't aim camera's up in the air at all if you can help it (it's a waste of camera capacity).
The cameras around the pentagon were probably 5+ years old (even high security facilities don't update their cameras every year or two, as besides anything else it creates issues with security during the works
), based on even older but proven reliable technology, including the storage.
It wasn't expected to have a threat from an aircraft, and the actual road and foot approaches would have had the coverage, it's probably sheer luck any of the cameras caught anything usable of the aircraft given it was something so far outside what they were intended for.