Those are "how" questions; "why" is too much into the religious and philosophical realm, thus too far outside science to be fitting for this forum. But it is important enough to mention, IMO, lest we assume it's not, at least for many, if not most.Only a dumb species like humans would ask why? There are too many potential answers. Results of collisions in protoplanet formation, types of star, distances from their star . . . . . . . . . . . .
[Is that semicolon the best use of grammar? It's another weak area for me.]
Yes. Using 10^23 for the rough estimate for planets in the observable universe, then, say, 10 billion life-lovin' exoplanets would be 1 / 10,000,000,000,000 the fraction of planets. Given how accustomed we are with working with odds, it would sure be odd if that is a high fraction.But I do suggest that the numbers have to stack up in the billions in favour of 'low' life forms (as in less evolved) and, at a guess, millions of 'higher' forms of life.