Many of the most popular (cheaper) pens are heavy things made out of metal. The Pilot Metropolitan, almost everything Jinhao, the Baoer 388, aluminum versions of the Safari and Kaweco Sport, etc. They don't seem to have issues with corrosion or rust, as long as they aren't used as an eyedropper. They are sturdy and resistant to shattering.

Yet, in the past, fountain pens made of metal like this are extremely uncommon. The earliest fountain pens were made out of hard rubber, and expensive models were gold fill or solid gold. By the 1940s, nearly every pen was made out of plastics. Plastics at the time were difficult to work with. Celluloid is flammable, fragile, and prone to decay with time. Other plastics used are a problem for pen repairers today. The plastic used in vintage Waterman and Eversharp pens is fragile and easily breaks during repair. The Parker 21 is also known for being made of fragile plastic. Esterbrook seems to be a rarity in that the plastic they used was strong and resisted decay.

Why didn't they make pens out of metal? I'm excluding gold fill pens, as these fell out of fashion, and solid gold pens were far too expensive for most people. I don't think I've ever seen a vintage pen that fit the now common construction of metal body with a shiny coating on the exterior. Was it not economically viable for some reason? Were heavy pens considered clunky or hard to write with? Did advances in metallurgy only allow this to be possible recently?