brainwashing!
...might have something to do with it! Lufthansa also only hires ab-initio pilots from their own pilots school. As a reason they state that it is easier to train someone from the beginning than to familiarise someone already flying with company policy. Also, it allows them to maintain quality of training to their own standards and gives them a heads up about someone not able to perform to standard before they even get in the rh seat of an airliner...
...out of an economical sense I've never undestood it either!