Because any of these law-abiding people owning a gun could snap some day and go rogue? I'd say that's a really good reason to reduce the guns availability.
Taking your garbage edit of the parent comment and setting it totally aside:
Legal firearms routinely become illegal, most often when they are stolen by people who are already criminals. Unlocked cars are a great source of 'legal' pistols as it turns out.
The problem of small arms proliferation is real and it is one of the reasons U.S. and other modern nations regulate them so much.
If your law abiding neighbor is building bombs in his garage, is that a problem?