A study done in 1973 by Peterson reported the following:
Contributions to baseball velocity calculated over a 9.15m distance immediately following release;
Legs and Hips 37%
Trunk and Shoulder Rotation 15%
Arm strength contributes to 48% of a pitchers throwing velocity.
If you want to be able to use the power developed by the larger muscles of the legs, hips and torso; the shoulder, arm, forearm, wrist and fingers must be stronger relative to the body.
In other words, arm strength is more important than core strength because the arm has less muscle mass than the trunk.