From my understanding Americans are far less concerned about the laws in the constitution and why it was written than they are about how the laws can help them in a given situation.

Some of the amendments are very good like the right to trial by jury and the protection against excessive fines or bail costs. Other's which were originally made with good intentions or in a society where it was important haven't aged as well like the right to bear arms and the prohibition of alcohol.

Specifically the second amendment commonly known as the right to bear arms is not as dry cut as it is often interpreted. The law in full reads:

"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

The funny thing about this law is that originally there was a lot of debate about it, many argued that the law was strictly orientated towards the rights of a militia, not an individual. It was finally ruled by the Supreme Court that the right to bear arms is orientated towards the individual, for the purpose of defending the freedom of the state.

On point though the second amendment was written into law because in 1791 this was a real concern and arming the people was an excellent way to stop it. Today there is no way that even a well armed group of individuals could stop the American government, they'd be completely overwhelmed. It's very possible that a lot of American politicians would agree because of this reason that it is an outdated law however for a politician there is a much greater concern, being re-elected.

A lot of people in America are very vocal in their defence of the second amendment, any politician who actively tried to remove the second amendment would face enormous public backlash and would likely lose their position in the next election. For that reason the politicians of America try to avoid this topic for fear of losing their job.

As to why so many Americans want to keep their guns, it would be too broad a stroke to generally put it under any one reason accurately but if I were asked to it would be because they like their guns and don't want them taken away.