I’ve stated before that I felt the SETI is a failure by design.
My reasoning is quite simple. In order to receive a radio signal from another star where the transmitter is operating at any reasonable power level, highly direction antennas must be used at both the receiving and transmitting station aimed at each other in order to have enough gain for a signal to be strong enough to be detectable.
High gain antennas have a very narrow beam width requiring that they be precisely aimed.
The SETI protocol requires several steps of confirmation. When a candidate carrier signal is received, the antenna is first pointed away, if the signal is still present then it is a terrestrial interference source.
Next another station remotely located is contacted and ask to confirm the signal, if the signal is still present, and received by that second station, at that point it is known to be extraterrestrial in nature.
Then, only if the signal is still present a day later, is it considered a viable extraterrestrial signal.
The Earth rotates, it is also orbiting the Sun, the Sun is orbiting the galactic center of gravity, and the entire galaxy is moving with respect to other galaxies. There is also relative motion between the Sun and other stars within our own Galaxy.
Likewise, another planet around another star harboring an alien civilization, is also likely rotating, and is orbiting a star, and that star will have relative motion to our own Sun.
Thus the potential for two narrow beam high gain antennas to remain aimed at each other for five minutes, let alone 24 hours, is exceedingly small, unless, the transmitting civilization is intentionally beaming a signal towards Earth, is aware of our positions, and is compensating for relative motions.
This seems unlikely unless alien civilizations have a means of detecting our existence other than radio.
So that is the first reason SETI won’t see anything even if the Universe is jam packed with advanced civilizations that still uses radio for communications.
There is another reason that SETI is unlikely to detect alien civilizations via their radio transmissions, and that is that the type of signal SETI is designed to detect is a type of signal that tends to be inefficient and thus not likely to be used in our own civilization much longer, let alone any more advanced civilization.
SETI only looks for very narrow band carriers, but carriers are a waste of energy, they are required only for proper decoding of a signal, and present day communications are leaning towards ultra-wide band spread spectrum techniques that would be completely undetectable by SETI.
Having a relatively small number of transmitters networked, as in wireless Internet, makes a lot more sense than having single transmitters for every type of program or transmission, and that seems also to be where we are headed.
The only signal we could detect, under existing SETI protocols, is one beamed intentionally at us, narrow band in nature, and lasting more than the 24 hour period required for confirmation.