The more dollars and jobs you can claim you brought to your district during the next election, the better.

This isn't just a NASA thing, a NASA contract is probably pretty small compared to getting a new military base, or keeping one open that the military wants to close.

Because US congress is made up of elected representatives, and each representative is incentivized to do so by the desire for re-election.

Because it benefits them (in kickbacks from contractors, for example) and because they can get away with it.

Having a well functioning space program? Doesn't benefit them much or at all.

Ultimately, it's because voters don't call them on their BS. If you've been a NASA fan, encouraging NASA funding regardless of what it's for, you're part of this problem.

I'm not an US citizen. I can't call out US politicians on anything because I don't have a vote.