It's because there isn't a proper bidding process. Politicians just give sweetheart bloated contracts to the same contractors every time, and those contractors then "donate to their campaign". If they instead used things like sealed bids and had contractors compete against eachother, it wouldn't cost $1B per mile for NYC's 2nd avenue subway, absurd.
NY Governor Cuomo surprisingly called this out recently, describing it as a "transit industrial complex": https://nypost.com/2019/01/04/mta-officials-are-fuming-over-...