Why are there 7 shoulder extensor muscles?

Best bet: evolution: The posterior deltoid, latissimus dorsi, teres major are shoulder extensors. Other muscles with extension moments include subscapularis, long head of triceps, infraspinatus, teres minor, sternal head of pectoralis major. Presumably shoulder muscles evolved to facilitate independent use of the upper extremities for fine motor tasks (allows me to type this answer) in upright biped organisms.

Post deltoid, lats +: Major extensors include the posterior deltoid, latissimus dorsi, teres major other muscles with extension moments include the subscapularis, long head of the triceps, infraspinatus, teres minor. The sternal head of the pectoralis major can have an extensor moment when the arm is in a forward flexed, internally rotated position.