Since McNamara’s cost plus contracting overhaul the primary skill set of every defense contractor has been strategic incompetence.
There is simply no incentive to succeed.
Fixed price contracts were tried in the past and they've failed as well. Basically, you're asking someone to guess just how long it will take them to invent something, then halfway through the government inevitably changes their mind and tells them to morph their invention into a different invention. It had problems.