It's an interesting theory but I feel like it doesn't reflect the reality. If galaxy was indeed teeming with rocks containing some sort of microbes than where d

It's an interesting theory but I feel like it doesn't reflect the reality. If galaxy was indeed teeming with rocks containing some sort of microbes than where did they all come from? It'll take an enormous amount of effort to populate an enormous amount of rocks and scatter them across the galaxy. I wouldn't presume that Protozoa would appear on asteroids just by itself. For this mechanism to work there would have to be millions or even billions of such rocks for every star system to ensure that at least a couple of them land on a ripe planet and some of the microbes survive the impact. Most of these rocks would end up falling into a star or a massive gas giant. And we have to think in terms of billions of years here. So these populated asteroids would have to keep falling on a potential life-supporting planet for a couple billions of years until it indeed become habitable. Ok, even if this mechanism of fertilizing planets was indeed functional as you suggest, we still don't see any evidence of intelligent life out there in our galaxy. Surely, out of billions of planets and billions of years time there would emerge at least a couple of civilizations capable of colonizing a significant part of our galaxy. There's certainly a slim possibility that we may be the first of such civilizations, but I'd be surprised to find out that humanity is THE smartest and most reasonable civilization out of billions others that our galaxy was capable to produce so far.