You are right, it's a perfectly valid hypothesis, but it is an almost useless one, for at least two reasons.

* It is improbable.

Say I hear what appears to be a human being, but whom I cannot identify. It may be Roger Waters, there's nothing supernatural here, there's no reason why it couldn't be him. Yet, there are so many other people around that this hypothesis is totally useless: it very likely isn't Roger Waters, so I'm better off without it.

* It is very hard to prove / disprove

Intelligent, technologically advanced beings could very probably generate a wide variety of signals, so it is hard to prove that a certain signal cannot be generate by them; we also have no idea how the "typical" ET signal looks like, so the hypothesis doesn't really help us in furthering our analysis.