SETI folks (both radio and optical) only claim to be able to detect DELIBERATE beacons purposefully broadcast to attract new members of the galactic club. Period. They freely admit that they cannot detect unintended transmissions, nevermind decrypt them just for the basic reasons you folks have cited. They are not stupid people. And they know damn well that if no old and powerful civilizations are NOT deliberately searching for newcommers, their assumption and effort fails. Your criticism misses the point.

I find the SETI premise flawed in that I think that the best evidence (paltry at that) we have indicates that animal life is very rare and intelligent animals with technology rarer still. But hey, go take a look and see if there are surprises in store...