Because we live in a secular society with freedom of religion, the church should not be involved in making laws. Of course the church can influence voters that are congregation members, but beyond that the separation of church and state exists in order to create a fair society for members of all religions, and to remove it would create an environment of religious discrimination.
Church is based on spirituality. Spirituality is not tangible. For example you can't prove god. You can't prove that soul's exist. Our world is physical and government rules physical events or things such territories, food regulations, schools (people learning things for our physical world). Church takes in account intangible things in a physical world.