or.. they wouldn't be able to survive here because they like a dry dead planet with almost no oxygen and water. Assuming the universe is engaged in this large scale panspermia, aren't we getting hit by alien microbes constantly? why would the ones we make be so much more dangerous?
Because mundane microbes wouldn't make a very good science fiction plot.
Seriously though, if we were constantly getting hit by alien microbes,we haven't found any yet, so constantly seems like a bit of a stretch. Panspermia on the whole seems entirely improbable. First, you need a planet on which the conditions for life exist. Next, you need some sort of cataclysmic event like a meteor impact which strikes the surface of the planet hard enough to eject a portion of the impacted planet. Then the life forms on that portion need to survive the rigors of space including radiation, lack of normal gravity, lack of food & water, etc., plus travel through gas clouds, possible impacts with other objects, and so on. The life forms would have to survive that most likely for millions of years and then the ejected portion would have to come into contact with another planet capable of supporting the same type of life forms. Then the life forms would have to survive the entry into the planet's atmosphere and impact with the planet.