1) The average author has no conception the abundance of elements in the universe or their economic value, as you've said about Universal Water Scarcity™.
2) Sketching out a collection of worlds which are inhabited solely because there are valuble rocks there is more plausiable than sketching out a collection of worlds which are inhabited because they are conveniently Earth-like (or worse, Edenic).
no subject
In any case, I have two related guesses:
1) The average author has no conception the abundance of elements in the universe or their economic value, as you've said about Universal Water Scarcity™.
2) Sketching out a collection of worlds which are inhabited solely because there are valuble rocks there is more plausiable than sketching out a collection of worlds which are inhabited because they are conveniently Earth-like (or worse, Edenic).