I'm not sure why everyone assumes extraterrestrial.

FTL travel is also the same bottleneck to most of theoretical time travel.

Given that almost no signals from Earth traveling at the speed of light would have reached far enough to have attracted new attention of alien life, if UFOs aren't from present day Earth, then there would only be two circumstances that seem probable:

(1) They were here for a very long time such that there was no need to have been alerted to emergent intelligence in the past few thousand years.

(2) The more advanced than present but fairly similar class of alleged technologies is not from anywhere else and it's simply more advanced because it literally is the same technology more advanced. Which would also partially explain the limited intervention.

Though I'm extremely skeptical it's actually anything outside the norm of local space or time. There's been a lot of manipulation of public opinion around this topic before, and that seems the most likely culprit for the present activity given historical results.