Chronic tightness is rarely one thing. It is usually a pattern that made sense once and was never given a reason to stop.
An old injury, a demanding season, a posture held through years of work — the body organizes itself around what it is asked to do most often, and then keeps the arrangement.
The reason it feels permanent is that it has become quiet. You stop noticing a shoulder that never fully lowers, the way you stop noticing a sound that never changes.
Nothing about that pattern is fixed in place. Tissue reorganizes when circumstances change, and the first circumstance to change is usually that someone finally looked at the whole pattern instead of the loudest part of it.