"If aliens exist, why haven't we seen/heard from them via radio emissions, etc?"

It's a matter of:

-language

- data protocol.

First, aliens will obviously have a different language. Why would you translate from English to an obscure tribal language to communicate with an isolated tribe you have no business talking to? I think aliens have better morals that to prosetlytize to us.

Imagine if a telegraph operator in 1945 started receiving machine language packets via TCP?

He would not be able to interpret the language, even if the packets easily translated to English. It would be discarded as noise.

We might be hearing alien transmissions right now. We dont know the language or protocol, nor should we.

Personal theory: Gamma Ray Bursts are dense packet transmission on the Galactic Broadcast Network (probably propaganda anyways)