3D printing the kit would be unreasonably time consuming, and 3D printers don't have successful prints each time, even the slightest change in the environment can cause warping in the plastic, which can cause a catastrophic failure.
Furthermore, the long term durability would certainly be significantly less than that of a properly manufactured steel version.
Most importantly though, once he does have the tools and dies needed for manufacture, it will be far cheaper per unit kit than it would be to print it, of course he'll have to get back his costs for said tooling, however he'll pump out far more units through traditional manufacturing than he could ever dream of printing, offsetting the initial costs.