You have put your finger on a key point of difference there. Airliners are developed at the manufacturers risk. There is a unit price at which they are offered for sale to airlines, with various optional extras which can be added in, mostly at a fixed cost.
The problem with so much public sector procurement is that they contract with contractors to produce the item in question. The clue is in the name: contractors. They are world class at coming up with whatever set of optics are necessary to get the contract signed, and world class at exploiting every clause to maximise their return thereafter.
Defence contracts are perfect for contractors as it is so rare for the customer not to change the specification part way through. Even where there has been effective competition before the contract is awarded, once it is underway, the contractor often has an effective monopoly over any changes.
Rope a dope, alive and well after all these years!