If they aren’t encrypted on purpose to obscure the communication, the other likelyhood is that the protocol used is indecipherable/undetectable to us.
It would be the same problem — radio communication is discovered/developed and is initially “in the clear” relative to their technology. Then, as the technology matures, the technology evolves purely in the interest of more efficient communication over longer distances and/or through barriers/obstacles (buildings, terrain, planets, stars).
So it could also be that the communication is not necessarily encrypted (although, concealing comms is rational in many use-cases), it could also be that the way the radio wave carries the information is too advanced for us, even if “they” think it is simplistic.
We know how to modulate an electromagnetic wave to convey information and we can alter that modulation in various ways; however, the bottom line is that encrypted or not, all we know how to do is modulate the waveform (amplitude or frequency) and that’s what we look for: An unnaturally modulated signal to indicate it was manipulated by a (presumably) intelligent source. But perhaps because we don’t know all things undiscovered, we could be getting blasted by signals and not even realize it. Yet.
However, if they were that advanced, were aware of us and had some interest in contacting us, I would think they would realize they needed to match our comms tech to make that contact. But if they aren’t aware of us and are just casting a net and fishing, it’s possible each of our signals are passing by each other completely undetected.
At least, that’s how it would seem to me.