People underestimate how dead and hostile the space is to life in general. First of all, you can just swipe most of the available galaxy space off the table because of radiation - the center of the Milky Way, close to our "core" black hole, is just pure hell. This is why we inhabit a remote neighborhood of the galaxy. You can extrapolate that most spiral galaxies are like that and only a small sliver of them are "in play".

Earth won multiple jackpot lotteries in a row, and that's only the variables we are aware of:

* Right-sized star

* Right-sized planet

* Not too far and not to close to the star

* Not too little and not too much water

* Oxygen

* Iron core (magnetic field protects from radiation)

* Right-sized moon (keeps rotation stable, tides)

* Gas giants absorbing a lot of debris coming our away

* A single-star system, with nearly circular, stable planetary orbits (very rare)* ... and so on

Yes, I know there are permutations that might also work, but in general it does not look good for spontaneous life.I like a theory that states that the Universe is just the right size where one single Earth has only a single probability to spring up randomly. The Maker covered their tracks well, if you believe in higher intelligence.