guess you were taught incorrectly.  here we are talking about a multi billon dollar and long term budget( project).  any preplanning cannot foresee all the challenges that may come up during the 20 years of construction.  you simply cannot compare your simple household budget to this type of project.  over time things change, prices change, etc. and you have to deal with them - you cannot say that oh, there is problem we did not foresee so let's just stop it and tear it down.  if such is the case, any long-term infrastructure project will never be completed, unless your overestimate the budget by 300% to cover any and all contingencies but this is not good planning.  to compare a simple short-term household budget to a long-term government project is ludicrous.