That is like presuming they must walk on two legs, breathe air, and eat plant or animal matter for sustenance.
Despite whatever the chairman of a university department of physics might say, that is narrow-minded, uncreative, trapped-inside-the-box thinking.
Despite the fact that humans have invented the iphone, the jet airplane, and the electric car, we are by all objective standards still a primitive life form. We haven't even developed a cure for male pattern baldness or the common cold.;-)
For us to point to our current "facts" as a determinant for things we have yet to discover or encounter is like listen to Lord Kelvin lecture us on the impossibility of video conferencing.
In all due respect, you are thinking like an accountant.
Linear thinking? Thinking out of the box? It would always be best not to confuse imagination with true insight. It is true that humans are perhaps the only species on earth that are capable of mystical thought and that mysticism or wonderment is at the core of scientific inquiry. However most of what is imagined or thought however emotionally gratifying is not real or reproducible. History is sodden with examples where musings and imagination lead to seductive delusions followed by human tragedy. When hard irrefutable evidence or reproducible experimentation is not present, a wise person does well to be skeptical.
So allow us skeptics to doubt that UFOs are alien visitors from many light years, dimensions or times far away or unknown or not perceptible to limited human senses and capabilities. Tolerate our “linear thinking”. Just don’t ask us to finance your fantasies with our taxes. As long as you don’t become dangerous, like many people who “thought out of the box”, feel free to go on imagining.
If you are interested, my own musings on what the UFO phenomena may represent has been posted on previous threads.