GB wrote:

I see them all the time - flat roofs that are completely level, and water
pools on the roof just waiting to get into the building. Why don't they get
built with a slight slope, maybe 1-2cms per metre? If that were done, the
worst of the water would run off rather than pooling and the roof would dry
out quickly once the rain stops.

Err they do ...

Of course, that's harder to build, but how much harder? Is the main issue
packing out the joists so the roof slopes, or is there more to it than that?

Search for "firring strips"