Because in real terms the majority of people aren't using them to break the law.
Figures suggest there is almost 1 gun to every one person in the states. (Pew Research Centre)
In real terms this is about 4 in 10 people.
Most people in the US look at Gun owners in an overall positive way.
For a lot people they have a function for a firearm other than just protection. Many use them for huting, sport or part of their job.
The majority of gun owners feel it is important to follow safe practices.
The availability of illegal guns are more likely to be contributing to crime than legally obtained guns are.
In 2016 there were 11,004 murder victims by firearm. (15,070 total).
As a percentage of the population that's 0.00336% killed by firearm.
So for the majority of law abiding gun owners there isn't a need to have them banned.