Because without some rudimentary numbers like the ones he's talking about, everyone involved can escape accountability -- the politicians, the contractors, the public (since they can claim to have been "misled" and not feel guilty later), ... But mainly because it's an invitation to corruption, for people known to be highly corruptible.

You're right that cost/benefit analysis is hard for transformative things like this, but the citizens throwing their hands up and saying "I don't care WHAT it costs" is like Pavlov's bell to politicians.