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.