Most if not all of the agile software development processes are based on the assumption that it is hardly possible or not possible at all to predict the real requirements and real workload size at the beginning on the project. The whole idea of agility is to try implementing the most critical stuff, see how it goes, get customer feedback and adjust the close plans.

Such limitation of the upfront planning minimizes the amount of time wasted on never used overplanning and on developing features, that after later consideration happen to be wrongly understood or unnecessary at all. Also the detailed plan non-existence welcomes changes easier. If there are no planning results to modify, there is no temptation to just follow the obsolete plans.