I assume because there is a feeling that the footage is not faked (as footage of UFO's tend to be), and because the military's incentives to fake a UFO video are usually very much in the opposite direction (i.e. nothing to see here, carry on).
I actually think it's perfect. Imagine they have future tech that absolutely crushes any current or next-gen competitors. The public is nosey and every year becoming more and more able to record high quality footage.
So let's drum up the "crazy" about UFOs. So when real UFOs are seen (the ones we created and want to keep secret) nobody can get a serious dialogue going because the UFO signal-to-noise ratio is awful. Everyone will focus on how crazy anyone sounds when they claim they have UFO footage. Especially when one of our military partners or adversaries manage to snag some airborne or satellite imagery of it.
What important news is getting buried? ;)