Because a domestic covert spying agency is a secret government. See the Stasi, or the Church Committee hearings, or the Iran-Contra hearings.
A domestic spy agency would spend all of its time trying to control government, strategic industries, and self-dealing. There are no checks on an organization like the CIA other than the old limitation that it couldn't target US citizens. There's no way to oversee an agency that isn't obligated to explain itself to Congress, is supplied by secret budgets, and information on whose outputs is protected by the Espionage Act (or ultimately, indefinite imprisonment and torture, threats made to family members, etc.)
We're sacrificing democracy for a secret hierarchy of secret police. You've cut government completely out of the equation if you have a government department with the latitude and ability to control domestic politics. It's basically a military coup.
A domestic spy agency would spend all of its time trying to control government, strategic industries, and self-dealing. There are no checks on an organization like the CIA other than the old limitation that it couldn't target US citizens. There's no way to oversee an agency that isn't obligated to explain itself to Congress, is supplied by secret budgets, and information on whose outputs is protected by the Espionage Act (or ultimately, indefinite imprisonment and torture, threats made to family members, etc.)
We're sacrificing democracy for a secret hierarchy of secret police. You've cut government completely out of the equation if you have a government department with the latitude and ability to control domestic politics. It's basically a military coup.