If, for example, humans had an average lifespan of, say, 30 years, and we reached sexual maturity at age 10, then we could "evolve" more quickly and theoretically "weed-out" more genetic diseases because we would live for shorter times, hence more natural selection.
Why don't we continually evolve for shorter lifespans (or do we)?
Extremely briefly, we do not evolve shorter lifespans because natural selection does not act for the good of the species.
As an interesting historical aside, August Weismann proposed essentially the idea you are suggesting in 1889, in his Essays upon heredity and kindred biological problems. Within a few years, however, he backed away from this hypothesis.
