Personal opinions, speculation, belief. Just like most of us, mathematically it almost a fact that there must be something out there. But nothing to really prove it.
Firstly, some of those quoted above are Physicists/Mathematicians, scientists. If it were you and a few others saying what these people said, yes, I would agree with you and label those opinions as "opinions, speculation, belief". But those quoted above are not like most of you and their fields of study has led them deeper than most of you in the understanding of the universe and reality.
Do not confuse your opinion and the value it has with the opinions of those quoted.
On the other hand...
As per our understanding of Physics today, our understanding of the Universe, it is not hard nor unsafe to say UFOs must be of terrestrial origin (some top secret weapons or aircraft operated by the US/Russians/British/whoever). Our science contradicts (apparently, at least) the extraterrestrial origin of these objects.
But right there lies the crux. Our current science and understanding of the universe. We cannot know what advanced civilizations might do, what their purposes would be, what means they have to traverse vast distances (from a mere 4 ly to dozens of parsecs) in a very short time (by means of wormhole physics or some other means).
As for the proof...
One thing is to be skeptic about these (although, how can any non-science oriented person be skeptic about something he/she understands almost zero...) objects and events, quite another is to ignore reports made public my several governments and government agencies throughout the world. Proof? There is proof that something inexplicable is happening or has happened or has been happening around the world. Even if we consider 5% of cases to be really inexplicable out of the thousands of observations/events, we get hundreds, if not thousands, of cases which defy science, technology and knowledge of the 20th and 21th centuries, which is why several scientists and reputable personalities demand the truth be told.
But hey...swallow whatever the mainstream media throws and pretend there are no governments/military/journalists/politicians stating UFOs are real.