Maybe because they haven't had success. If you are going to make that argument, than you can question the legitimacy of the Ravens #2, Patriots #3, Panthers #4, Titans #5, and Steelers #6 run defenses.
I disagree. I think the Cowboys are legit as a run defense. They do not rush the passer all that great but they do swarm the ball carrier. They haven't given up a 100 yard rusher this year.