The main functional purpose of a roof is to insulate — to stop anything from coming in to your house (rain, snow, animals, insects, heat, cold), and stop heat or cool from escaping your house. Why then does it make sense to make a roof out of thousands of little porous pieces instead of a couple big pieces? To keep the profits up of roof maintenance and installation companies of course!
According to a government study by researchers at Lawrence Berkeley National Laboratory, fiberglass shingles have around a 50% market share in residential roofing. Yet fiberglass shingles require the most maintenance, have the shortest lifespan, and provide the worst insulation of any of the three most popular roofing materials available.
It's simply common sense that a ton of little tiles slabbed over each other are not going to protect a structure from the elements as well as a few big slabs of metal. So, why do homeowners keep choosing fiber glass tiles? Well, they do initially cost less than the other materials to install (but this would probably change if the same economies of scale were applied to the other materials), and some do say they subjectively 'look better' than metal.
But, metal roofing can be designed to look almost any way one likes. In one man's opinion, the fiberglass roofing phenomena is a lot of group think on the part of homeowners and a lot of roofing businesses' self interest at play.