Or they started signalling around the time of the dinosaurs, and gave up around the time we invented the spear.

Or, perhaps they've signalled, we've received it, yet our protocols don't allow us to acknowledge it as artificial in origin because it was very brief (something akin to the Wow! signal).

One might say it's ridiculous to think they'd bother at all and broadcast so briefly, and that may be so, but there is precedent. Our very own Arecibo message was broadcast for less than 3 minutes. Would that be enough for an alien astronomer to contact other observatories on his planet to get confirmation?

(Disclaimer: I'm making no claims regarding the Wow! signal. Just pointing out that if we were to receive something like the Arecibo message, our own SETI protocols would not allow us to acknowledge it as being artificial in origin.)