Right, because computers are something you can make in your back yard. Don't be dense.
The vast majority of people lack the expertise to build or program computers which would be the actual parallel in this bizarre metaphor you've drawn up.
Not a very apt comparison, is it?
While (back in the day, and even now) building a computer from scratch requires at least an EE level of education plus a crap-ton of actual CS experience, making a working firearm requires a whole lot less.
After all, humanity has been making the things with crap technology since what, the 13th Century or so (counting cannons)?
The whole idea behind a homemade firearm boils down to materials (that can handle upwards of 40,000 psi or more), tight tolerances (so the gases are directed towards pushing the bullet out instead of leaking back through the receiver or blowing past the bullet out the barrel), and a bit of mechanical engineering (so you can build a reliable trigger, a reliable extraction mechanism, prevent jams, etc).
It's something like a whole order of magnitude less complex than building a whole computer from, say, Mouser/RadioShack parts.Hell, zip guns have been around for years, and it doesn't take any specialized kind of rocket science to 3D-print a firearm that allows a metal sleeve for the barrel (or has the facility to screw a barrel into it), or make one sufficiently useful for one shot.