Because it's not nothing in the cylinders. You still have air being compressed. just not as much. The compression is only a few lbs different. It's still 100+psi. What do you think causes the vacuum in the manifold. It's the pistons sucking the air in. You aren't sealing the cylinders off when the throttle closes. There is still some air and fuel being introduced into the cylinders which still gets compressed. Where is tremendous anmount of vacuum that's pulling the engine down located. If that much vacuum was being created it would suck the carb open which defeats your purpose. You are only making a very small amount of vacuum in the intake.