Because it's not a requirement, and most people are not intentionally or accidentally defrauding the government.

The issue is that there is no incentive to do the additional work necessary to generate reproducible results because of the pressure to constantly generate sufficiently novel results to publish.

If you spend the additional time required to have fully reproducible results and your competition is not, you're probably going to lose the game (where the game is obtaining more funding).

Not generating reproducible results doesn't mean you're a fraud, but the absence of a requirement to generate them in order to publish means that it's easier for fraudsters to operate that it would be with that requirement.