When "Life" is mentioned in this context, it doesn't mean green slime, but intelligent technological civilisations that can communicate.

Green slime (or blue or purple slime) are probably common in the Universe. So too may be fish and seaweed, but technology is difficult and requires a long-term stable world with the right ingredients for technology, and both oceans and land. This is not as simple as it sounds.

This rules out the galactic centre - too much radiation and supernovae, and also excludes the outer region - too little metallicity and therefore not enougj rock to build proper planets.

All very subjective, but seems sound.

Read Ward and Brownlee "Rare Earths"