Life critical software that gets visibility by congress tends to be a very bureaucratic process. Your boss doesn't want your commit being the one that causes a worldwide diplomatic issue.
I assume that smaller/cheaper drones avoid a lot of this because the stakes aren't near as high and quite a bit of the development occurs in private industry first.
I would guess that DoD procurement rules have more to do with it than Congress, but perhaps Congress defined DoD procurement rules.
Milspec is expensive and process heavy, see what a B52 replacement trash can costs, for just one example.
> visibility by congress tends to be a very bureaucratic process
See also SpaceX vs. NASA. No way would NASA have been allowed to blow up as many rockets as SpaceX did to finally get to their working solution.