I don’t know the official reason, if such a thing even exists. My heads screwed on just wrong enough to hazard a guess:

The empathy of inconveniencing and materially harming their constituents (or the fear of their electoral retribution) would be such a driving force that the government would seek to end any shutdown before it came to that.

Of course, any well-meaning intent withers in the face of monsters willing to kill, and let others die, for the facade of politics they don’t even truly subscribe to.