The three main factors are probably: 1) There is (just like with any bespoke product) a real risk that it won't be completed (at all / on time / for a profit).

The three main factors are probably: 1) There is (just like with any bespoke product) a real risk that it won't be completed (at all / on time / for a profit). 2) It's very capital-intensive (so even the biggest defense contractor is going to need to borrow). Even civilian ships cost tens or hundreds of millions of dollars. 3) The capital is tied up for a minimum of 1-2 years (depending on the size of the ship). Re 3: Even during WWII, with "all hands on deck" and less complex ships, the US wasn't able to build a fleet carrier in less than 24 months (authorization through commissioning). "Liberty ships" (mass-produced, unarmored transports) took about 2 months through commissioning. So the Navy would probably have to arrange some sort of loan guarantee... and now we're back to the supplier not having a strong incentive to do things right.