While I am horrified by the sums of money, I think there's a slightly less cynical take than the congressional districts one.

If you want to have an industry of people who make X, then there need to be people employed making X, every year, for decades. And at more than one company, so that it isn't too fragile a career path. When X is single-use rockets there is the advantage that after one flight you have to re-build, whereas when X is tanks (or other long-lasting hardware) you have to choose to throw the old ones away.