It is something called laws. The reason that DAL lets these girls fly is money. Simply put, if they did something that would disqualify them, it would have to be something physical. If that happened, then they will drain DAL sitting at home with their cats collecting disability.
The threat of them going out on medical en mass is a real issue. Disability for 4000 FA's would cost DAL a ton.
That in a nut shell is why DAL lets them fly till they die.