I'm certain that the vast majority of scientists, physicists for certain, do "believe" in extraterrestrial life. Belief makes it sound kind of quirky though. It's essentially a probability measurement. Is it more likely that we are the only life in the universe, or not? You reach the same conclusion whether by intuition or by what crude modeling we're capable of. It's not really possible to create a compelling scientific argument for the lack of alien life. This is why the Fermi Paradox is a thing.

The reason people try to discount aliens is because you can attribute literally anything to aliens. And so you reach another equally simplistic binary. You discover some unknown phenomena. It is aliens, or not? Well we've discovered a whole lot of unknown phenomena throughout the years (and centuries and millennia depending on how you want to play this game) and often times it really did look a whole lot like aliens, but to date it's never yet turned out to be them.

And since the determination of an unknown phenomena often comes down to probabilities, aliens get weighted down a good deal. But nonetheless, I have no doubt that at least lurking in the back of every physicist's mind is that something now as regular as a fast radio burst could indeed finally be those LGM - the kind of joking nickname given to the first radio pulsar discovery. Standing, of course, for little green men. Is it? Well probably not, but it'll always be probably not - including when it finally is.