Well, there are firearms out there that use cast parts. Ruger, especially tends to use a lot of castings.

But casting can be a finicky process and can sometimes leave unexpected and invisible voids and weak spots in the metal. It's not something you'd want to use on the pressure-bearing parts of a firearm unless A) the firearm was designed with that in mind and those parts are overbuilt to compensate for potential weaknesses, and B) you have a lot of experience with casting and you're well able to examine the parts and determine if they have hidden weaknesses (such as with x-ray analysis to detect defects).

Casting also usually leaves somewhat of a rough finish, so it usually needs finish machining done where mechanical parts need to interact with close tolerances. So casting is more of a starting point than a one-and-done manufacturing method. Casting can save you some effort when compared to starting with a solid block of metal.