And for the same reason that software estimates suck. The lower bound is quantifiable: if everything goes as expected, and this is all the work, then project t

And for the same reason that software estimates suck. The lower bound is quantifiable: if everything goes as expected, and this is all the work, then project time is the sum of each piece's time. But the upper bound is unknowable. If a thing that we don't know about happens, how long will it take to solve, if a solution is possible? And what portion of the project will it involve? But that's really a gripe about PMs not understanding compositions of multiple normal distributions.