A hypothesis that makes testable predictions is perfectly valid science even if it involves aliens. "Aliens did it" in itself is not a scientific explanation. Aluen life is probable a priori just given the size of the universe; unfortunately we also have all sorts of other evidence that technological alien civilizations are nowhere near us.
With SETI, we explicitly make the assumption that any prospective aliens are using communications technology that can be intercepted by us.
That's what I suspect. From what I understand, we wouldn't be able to detect our own radio broadcasts from more than half a light year away. The only realistic way in which we might detect alien radio signals is if they were either really close, or they're using a focused beam aimed directly at us. Which would imply they know we're here and listening. It all seems a bit unlikely.
Detecting alien megastructures, though, there might be something to that.
With SETI, we explicitly make the assumption that any prospective aliens are using communications technology that can be intercepted by us.