I would guess it is because there was no evolutionary pressure to have joints that can sustain 80 years if there are other parts of body that only sustain 18. Maybe I am missing something but I would expect that life expectancy of various parts of body of each specie converge because of that.
Evolution is rarely so tidy in its convergence. Humans have vestigial organs and crazy anatomical features left over from our ancestors. In light of that it would be a rather stunning coincidence to find that when it comes to aging across species (!) that organisms seem to have all their systems age at the same rate simply by individual evolution of each of those systems or that all the systems are effectively immortal with nothing in-between.
Without a coherent theory of senescence this is all conjecture, but there's compelling evidence that there's some set of hidden variables we aren't seeing that is "pulling the strings" when it comes to aging.
On reflection evidence is too strong a word given that we don't have a good, specific hypothesis for what's going on. Observations of the sort I laid out above.
