Waiting for cement to fully settle and solidify presents a number of problems, both from a point of view of logistics and planning and a civil engineering point of view. It can mean that you need to wait and halt building work, sometimes for a very long time.
There are a number of factors which need to be considered when laying out cement, such as what else needs to be done on the construction project and ensuring that work can still be done while the cement is drying, and how long this drying period will last.
Cement dries at a speed dependant upon how much there is of it and how much moisture it contains. In some of the biggest building projects undertaken the amount of cement could potentially months or even years to fully settle, although methods such as laying the cement in sections can help to alleviate this to a degree.