I suspect it's just "time". The universe is what, 13.8 billion years old and give or take? It took 4 billion years or so for life to evolve here, so if you apply the same rules, life may have been "out there" for around 8 billion years. But how long have we actually been looking in anger with radio telescopes, maybe a 100 years? Out of that 100 years the technology to scan at speed and sensitivity required 10-20 years? Also we can only listen for powerful radio sources, so we've only really been looking/listening for less than the blink of an eye in real terms. Plus the universe is so vast (if there's even only 1) that it's akin to looking for a grain of sand on all the beaches in our solar system. I think it's just wishful thinking that we will "look" and find something within 1-2 lifetimes. It might take a 1000 lifetimes but I'm sure one day we will find something. Or it will find us if it's hasn't already!
Also I think our understanding of the Universe has reached the point where many things may never be "tested" as such, and will remain as theories for decades to come. We simply don't have the time, money or knowledge to build something to see or measure things in the material world. Even now I think they are saying the LHC isn't powerful enough to go to the next levels that theories predict. Add to that the discoveries in the quantum world aren't intuitive or even predictable in the same way discoveries were a couple of hundred years ago. The things we may discover or predict may well be beyond out ability to understand them. It could be that 99.999999999% of everything out there is hidden in dimensions that we will never be able to physically observe.