Why does cement take so long to set?: As Uwe Waibel commented, it is not cement but cement concrete what may take a relatively long time to set.
Well, you can always use carbon sulfate aluminate (CSA) cement if you need quicker setting times.
Of course, you have to pay more for CSA, which is the reason a majority prefer to plan for normal setting times of one week and do the work in parts in such a way that setting times impact very little final project times.
This happens because, believe it or not, structural work is just a small bit of all the activities you need to complete a building.
In the end, the project time is the same, setting or not setting, only that piping, electric networks, wall and floor finishing, tiles, sewer, bathrooms are one week delayed, so the whole project takes one week more, which is the time you need to wait for the first batch of cement to finish setting.
Meanwhile there is plenty of work to do.
That does not take in account the heavy work of concrete which is not pouring concrete or waiting for it to set but rebars.
Compared with doing this, pouring and setting times are peanuts