If expansionary policy produces high GDP and low unemployment, why wouldn't the Fed always use expansionary policy?

There are limits to how long an expansion is successful.

Explanation:

Expansionist policies often lead to fake bubbles of market security where people invest more than they are comfortable with and a small slowdown can cause a market crash as they panic.

This is because of expansionist policies being used for too long

These policies are like a turbo get in that they are to be used in moderation because if one relies on them too often they can dis balance the economy by producing a lot of deficit in the reserves and also less governmental control.