I always liked the idea that there's a reason not to be visible; pretty sure I saw it mentioned on HN first, it went something along the lines of us sending out messages and getting one back that says "SHH! They'll hear you."
Radio emissions are actually much harder to detect because they're weaker than all the light bouncing off the planet. Aliens aren't picking up TV or radio broadcasts. Those become undetectable a little outside the solar system. The signal strength fades (due to the inverse square law) to under natural radio background a little into the Oort Cloud. Only very deliberate high power and highly directional signals can be detectable at interstellar ranges.
The Dark Forest theory is just silly and I'd go so far as to say pseudoscience. It just doesn't make any sense if you think about it for a few minutes.
Could be that ethics and philosophy ends up dictating that you stay out of it. There's speculation that they've sent probes.
Could be they have little to gain from contact, and prefer to leave us to our own devices. After all, it is the right thing to do.