Because we entrust certain actors to be the enforcers of laws in society. This is the way societies have worked since the dawn of time; we aren't all charged with upholding laws, we give that task out to people to have that be their job.

This is in fact not how "societies have worked since the dawn of time".[1] The innovation of the monopoly of use of violence by the state is a relatively recent one, and not demonstrably necessary or sufficient to maintain an orderly and just society.[2]

[1] https://en.wikipedia.org/wiki/Police#History

[2] https://en.wikipedia.org/wiki/Monopoly_on_violence

The 2nd amendment was added based on how societies have worked since the dawn of time. Keep in mind that even in our "modern" era ~100,000,000 people were murdered by their own governments in the 20th century.