Probably has to do with liability, warranty and money. If you install it yourself, and have an issue, some people will try to blame the Nitro Piston as being faulty even if they messed up the installation somehow. Most manufacturers will void the warranty on a gun if it's disassembled, unless it's done by a qualified authorized repair center. Even though quite a few airgunners are very knowledgeable and could do the install them selves, they still want a service center to do it. That forces you to pay them for the install, helping to generate more money, or buy a rifle with a nitro piston already installed(again more guns sales and more money).
Nathan