I personally don't know if there ever was a video of the impact other than that of the security camera [0]. I think of two reasons: the Pentagon is not NY, which is a highly populated area in a developed country, it's natural that so many people were filming. Even more so, the Twin Towers were cultural icons. I don't know if many people were around the Pentagon, but I have read about witnesses of the crash in DC. The other reason would be that they don't want to release it and it's understandable.

[0] https://en.wikipedia.org/wiki/The_Pentagon#11_September_2001...