A surgeon fixes the structure: the torn cuff, the worn joint surface, the compressed nerve. Your body still has to be talked back into using it. Swelling and pain shut muscles down, and it happens fast. After knee surgery the quad often will not fire properly for weeks. After shoulder surgery the capsule tightens while the arm sits in a sling. Scar tissue lays down in whatever position you hold. Meanwhile the weak hip or the stiff shoulder blade that helped cause the problem is exactly as weak and as stiff as it was before you went in.
That is why waiting it out leaves people stuck. Motion is easiest to win back early, and harder every month after that. Pain settles into new habits: a limp, a hitch, a shoulder that lifts by shrugging. Prehab attacks the same problem from the front end. Going in with more motion and more strength gives you more to work with on the other side. Either way, the plan starts with an evaluation that looks for the cause, and it ends at a defined endpoint.