Polymer handguns are EASILY the most profitable market in firearms today; and has been for at least 25 years now. The profit margins are VERY thick on polymer guns, typically over 100%. Those profits are thinning as times goes on and more makers get into the game...but even if they cut the profit margin by 50%, those guns would still be the most profitable.
When you're forging and milling, it takes time and expensive equipment. It also produces expensive consumables in the production process in the form of milling bits, polishing pads, and cutting fluid. Polymer guns are 85% injection molded, stamped; that's cheap to make. Molding and stamping are also extremely precise methods of manufacturing, so the guns tend to have less issues and fewer returns.
