More software projects have gone awry for lack of calendar time than for all other causes combined. Why is this cause of disaster so common?
First, our techniques of estimating are poorly developed. More seriously, they reflect an unvoiced assumption which is quite un- true, i.e., that all will go well.
Second, our estimating techniques fallaciously confuse effort with progress, hiding the assumption that men and months are interchangeable.
Third, because we are uncertain of our estimates, software managers often lack the courteous stubbornness of Antoine's chef.
Fourth, schedule progress is poorly monitored. Techniques proven and routine in other engineering disciplines are considered radical innovations in software engineering.
Fifth, when schedule slippage is recognized, the natural (and traditional) response is to add manpower. Like dousing a fire with gasoline, this makes matters worse, much worse. More fire re- quires more gasoline, and thus begins a regenerative cycle which ends in disaster.

Frederick P. Brooks, Jr., The Mythical Man-Month

https://web.eecs.umich.edu/~weimerw/2018-481/readings/mythical-man-month.pdf