For the same reason we can't have pick-proof locks. Total security is physically impossible. You have to defend every access surface with equal flawlessness. At

For the same reason we can't have pick-proof locks. Total security is physically impossible. You have to defend every access surface with equal flawlessness. Attackers can all coordinate on a single point of failure. By nature of this equation, any time you make a part more secure, attackers will simply focus their efforts on the next weakest link and go at it again. Immunity can't prevent disease from entering your body either; They can only make it harder and actively respond to intrusion.