Early outdoor gear had its roots in the military where coats were primarily made out of wool and natural fibers. Prior to that, people had used animal skins, relying on a system of trapping air to keep themselves warm. If you wanted to stay warm outside, you needed to pile on lots of thick layers and bulky garments.
Older rain slickers were coated in polyurethane, which was not only heavy but poorly ventilated.
As technology evolved, however, brands began developing ways of making fabrics more lightweight and compact. Waterproof membranes
emerged that could be put on textiles, cutting down on the weight and making the material softer and more breathable.