1) The requirements aren't levied by congress. They are created by NASA civil servants who are usually the technical experts in a particular field. These aren't people who have much of any interface with lobbyists.
2) A lot of the instances where it's not worth it to suppliers isn't because they are being boxed out by some industry giant clamoring for some giant contract. It's usually closer to "it's not worth our effort to overhaul our manufacturing process to meet requirements so we can sell NASA a $70 teflon seal." The giant corps have little desire here either. But being a big manufacturer is sometimes correlated with having a more mature manufacturing process that meets specific standards.
There’s already enough corruption in govt, we don’t have to make believe it’s in places that’s it’s not.
The political risk is the more salient point to me, and jobs is just a way to mitigate it. (You could also, for example, mitigate it with less productive means like lobbying)
(Suppliers is a different story. A lot of time NASA is handcuffed by which suppliers actually want work with them. There’s a lot of hoops to jump through and many suppliers just don’t find it worth the hassle)