Because it makes a lot of sense. If microbes can survive long periods of hibernation, then statistically there is a good chance some of them hopped a ride to earth. We get hit by 20 tons of debris every single day. It's not an outlandish theory or anything.

The poetic reflectance of nature's preferred method of reproduction (chemical delivery of ingredients to other ingredients) is a lot to turn down in the absence of better theories. Plus, these compounds had to come from somewhere, and we can't find any mechanism for creating them on Earth before life existed, combine that with the fact that we've found literally every single building block needed for life, to exist on astral bodies outside of the planet... It's a lot to dismiss.