Refer to figure.
Flaps are usually not fully extended at takeoff for several reasons:

1. Increased Drag: Fully extending the flaps creates more drag, which can reduce the aircraft's acceleration and takeoff performance. A moderate flap setting balances lift and drag, optimizing the takeoff roll and allowing the aircraft to reach takeoff speed more efficiently.
2. Airfoil Efficiency: At lower speeds, moderate flap settings enhance the lift-to-drag ratio, helping the aircraft achieve takeoff speed with less drag. Fully extended flaps are generally used for landing, where increased lift and drag are beneficial for a slower approach speed and shorter landing distance.
3. Engine Performance: On takeoff, the aircraft needs to accelerate quickly. Excessive drag from fully extended flaps can strain the engines and reduce their performance during the critical phase of takeoff.
4. Runway Length: Takeoff distance is a crucial factor. Using a moderate flap setting helps optimize the runway length needed for takeoff, ensuring that the aircraft can safely become airborne within the available distance.

By using flaps judiciously during takeoff, pilots can achieve the best performance while managing drag and engine efficiency. Therefore, flaps fully extended is creating to much drag and not efficient for take off.