Lets be honest here. What would you do when you've dug a hole in the middle of Boston and disrupted traffic and every other piece of infrastructure in that part of city after you run out of budget? Walk away and leaving a gaping pit? Spend more money and fill the pit in, and then be left with your original problem, that's now probably even worse than when you started? Fill in the hole, and then dig a new hole? Or do you effectively throw good money after bad and finish the job?

Comparing a government transportation project to anything in the private sector is a canard. There's literally nothing comparable. That isn't the say that government contracting policies and processes are perfect and not abused. They clearly aren't perfect, and they clearly are abused. See the F-35, and well, pretty much every military purchase since the Cold War. See the vast majority of government infrastructure projects. Most importantly, these cost overruns aren't something that's fundamental about private-vs-public, but rather something about how American governments function.[0] Unfortunately, there's no single smoking gun, and instead everyone falls back to their favorite boogeymen. (My favorites are multiple environmental impact studies, public comment periods, fraud, and of course willful incompetence (i.e. "I don't believe the government should do this, so I"m going to do a really shitty job, and then use my poor performance as evidence that the government is intrinsically incompetent."))

[0] https://www.bloomberg.com/view/articles/2016-04-08/why-u-s-i...