Actually, even humans do not intentionally broadcast across space using the EM spectrum unless it is as narrow and directional as possible.
SETI is looking for EM transmissions that reach us.
There would be two possible sources for such signals, a civilization that is sending signals to some other civilization that is within the beam width when the signal gets to us (or was in the beam at some time between when the signal originated and it arrived at our location) OR ...
... like the signals radiating out from earth, waste artifacts of local communication.
Most of the EM transmissions originating on earth are that part of old radio and TV signals that did not get reflected off the ionosphere as intended and so escaped to propagate through space.
As soon as we were able we changed our transmission procedure to make EM transmissions as directional as possible.
Which leads to a third possibility. With our current technology, we are on the threshold of being able to detect extrasolar Earth-like planets. If an extraterrestrial intelligence is barely able to determine the existence of terrestrial planets in our solar system, we ourselves could conceivably be one of their intentional targets of a directed continuous narrow-band transmission, presumably designed by them to get our attention somehow (as a precursor to actual communication). One of the reasons for SETI research is to either detect the existence of such easy-to-recognize signaling, or to rule it out.
So far, we can confidently say (pretty much) that the sky does not seem to be littered with such attention-getting beacons.