Because legislatures often don't want to come out and admit to what they're doing, so they try to obfuscate it.
My question remains: if the legislature's definition of "firearm" included alcoholic beverages, should a judge allow someone to be prosecuted for unlawful possession of a "firearm" because he possessed a six-pack of beer? Or should a judge rule that regardless of what a legislature may decree, there is no way that a six-pack of beer can be so construed?
I don't want judges to be the final answer on what is a gun and what isn't. Do you? Do you want an unelected judge ruling that a handgun is not protected from infringement since, by his definition, "arms" are limited to long guns (rifles and shotguns)?
Let the legislature define firearms. If the people who elected them to office disagree, they'll let them know.