Because we have all the science figured out, and travelling those distances is impossible.
Did you ever image that there could be societies that first harnessed electricity 2 million years ago? - making us .0000001% as technologically advanced as them.
Well?
There probably are many advanced civilizations. And the chances of them even being in our galaxy are probably close to zero.
If it takes them 3 million years to get here, most likely not a problem. But if they can get here only 1 million years after inventing electricity, then we have a big problem.