Flat roofing is a pretty popular roofing style, but it does have its problems. People have used slanted roofs for thousands of years for a reason; water tends to slide downhill; something that a flat roof just can’t do.
If the roof is perfectly flat, then the layer of water that remains on the roof is unlikely to cause significant damage (although some is still likely), but flat roofs are rarely perfectly flat, and any slight indentation can allow puddles of water to form, which will cause damage far more quickly.
There seems to be no way around this issue, other than to abandon flat roof systems altogether. There is some hope though as roofing technology is developing all of the time, particularly in the areas of waterproofing and durability, and we could potentially see more flat roofing solutions able to tout lifespans equal to that of the more common slanted roofing systems.