If we are sure that our enemys won't attack us we can pare defense down to the minimum that ensures this.
If we are certain that the government will not save us from starving , we will work and save and buy insurance and contribute to charitys.
(even)If we are certain that the government can get suffecient revenue from a simple tax system , we cannot pry from the Congress the power to controll and kill and micromanage that a complex tax system provides to them.