Because trusting the government to have the power to determine who gets to defend themselves means that African Americans in the post-civil-war south, and Jews in 1930s Germany, don't get to defend themselves.

People keep saying that the US is on the verge of being a fascist white supremacist state - why do you want to give that state the right to choose who gets guns?

Edit: and in more immediately practical consequences, because gun-hostile governments have a history of publicly leaking the list of people who own guns.