Why is church and government separated?

In some countries, church aka religion is part of government policies and in some countries a particular religion is called the "official religion" of these nations.

In Western nations, religion normally plays no role in government practices and policies. In the United States Constitution, the First Amendment states that the government cannot establish an "official" religion, nor interfere with the practice of different religions among its citizens. There is a good reason for the amendment to forbid government interference in religion, it prevents government from denying citizens part of their freedoms.