Because they aren't possible?
Or, in other words, there is a wall.
becasue they have populated the other half of the galaxy?
And by a surprising coincidence, they are the only other life form on the galaxy, and they appeared excatly in sync with us? (Ok, half a million years earlier.) Yes, that's possible, but hard to accept.
becasue they don't need to grow that fast?
And spite in the face of Dawinian evolution. Why don't they have to follow the laws of Nature that we know to be valid all around the galaxy?
becasue they have all been wiped out be a variety of event. Specifically wiped out faster then they can be built?
That's the wall explanation again.