Because the DBA knows that independent auditors will eventually catch up with him/her. Maybe not this audit cycle, but someday. Since the DBA's pay and bonuses are a lot less likely to be oriented to benefit cheating, unlike, say, a star sales rep, the DBA is among the least likely to be untrustworthy.

At some point there just isn't a next level of watchers to watch the watchers, so don't let the paranoia keep your organization from functioning.