The best windows in Europe are made with thermally broken wood frames, but the best windows in North America have fiberglass frames. The European wood windows have my vote for the best made windows in the world. I consider them art. However, I have to admit that fiberglass is a pretty ideal material for window frames.
As wood takes on water and dries it can warp. If it takes on enough water it can rot. Fiberglass, on the other hand is impervious.
Expansion and contraction are another problem. Aluminum frames expand about 3 times as much as glass when they are heated and vinyl expands 7 times as much as glass. Wood does not expand when heated. It expands with humidity. The varying expansion rates can result in stressed frames, broken seals, and inoperable hardware, but fiberglass frames have nearly the same coefficient of expansion that plate glass does.
Fiberglass frames are made by a process called pultrusion, a combination of the words ‘pull’ and ‘extrusion’. Vinyl and aluminum frames are extruded, meaning the material is melted and forced thru a die. With pultrusion, continuous glass fibers are pulled thru a resin impregnation bath and then a heated die where the resin sets into the desired shape. Here’s a pultrusion machine in action: