I think it very likely that life exists elsewhere in our galaxy (let alone the universe!)
The discovery that the basic building blocks of life probably exist in comets, evidence that major impacts may “splatter” material from one planet to others (ex. Mars meteorites found on Earth), the staggering numbers of stars, planets, etc, the unscientific but seductive logic that if it could happen once, it could happen many times … and so on.
Other possibilities for failing to detect EM signals (in addition to than what other posters have contributed) occur to me. These thoughts are probably not original, in which case I apologize in advance to whoever stated them first.
1) Other life exists, it just has not developed along the same technological lines as us. They are not sending out detectible EM signals.
2) Perhaps every life form / civilization that does develop along the same technological lines as us destroys itself in fairly short order. Not necessarily with weapons of mass destruction, but simply through overpopulation, pollution, destruction of environment, disease. Depressing, or what?
Is SETI a waste of resources? From an academic point of view, (or to be able to say “In your face” to the (fundamentalist) religionists if other life was detected), perhaps. Given the distances and time scales involved, two way communication is impractical / impossible.
Unless, of course, they are sending us the plans for that Warp 10 engine …
Yah, that’s it. “To serve man”, only they are too lazy to pick up. They want delivery …