And for UFOs-as-ETs, these intelligences would have had to spawn pretty near our solar system, and their existence overlap in time with mankind's tiny fraction of existence (truly a blip in the lifespan of the universe), and detect our existence (or at least our system as a worthwhile destination), and develop tech to take them here, and be interested in this undertaking, and have the resources for it, and not wipe themselves out in an accident, ecological disaster or global war, and after all of this choose to just do mysterious flybys and abductions and no formal contact with Earth.
Too many "and"s for me...