What I'm not understanding is how it's more profitable to involve a 3rd party company, who will be taking their costs and adding a profit margin, rather than do

What I'm not understanding is how it's more profitable to involve a 3rd party company, who will be taking their costs and adding a profit margin, rather than doing it all in-house? If it costs on average $X/month to maintain a plane, then the maintenance company is going to charge you $(X+Y)/month, where Y is a decent profit margin. Certainly you'd save money by not involving the third party, right? Or are these companies happy to pay it because that $Y also covers risk of a sudden expensive repair?