Because that's how the incentives align. The goal is to cut costs, the cost of outsourced training pilots + the margin of the outsourced contractor will need to be lower than the cost of in-house training pilots.
Also because that's how pretty much all of outsourcing goes, it doesn't take a seer to realise that what's happened essentially every time is going to happen again. Even less so considering Boeing's recent history of cutting costs at the expense of safety and reliability.
