Actually, to correct you, that's like DPS ignoring survivability stats.. which, if I'm not mistaken, is what every last one of the bloody glass cannons does anyway by default.
Yeah, tanks do deal damage, but that's mostly built in for solo combat balance. We DO have to kill stuff on our own from time to time and can't have a monk follow us around to handstand everything to death (Or lancers pogo-sticking, or Bards being grossly fabulous). Even at 50% of the output of a dps class, that means you're only accountable for 1/5th of the total damage output in the party. .001% per-point increases in offensive stats are not going to mark any significant increase in performance in the grand scheme.
By comparison, there isn't a tank out there that lacks stories of entire runs been saved by the skin of a few points of extra hp.