These muscles become tight and shortened for a variety of reasons.

Everyone has different underlying genetics, structural make up, biomechanics and so on. These can be quite good to begin with or they can be particularly defective for a variety of reasons. We then do a variety of unhelpful things to our bodies over our lifetimes which affect what we were born with. Typically, people will have poor posture - all that endless hunching and slumping creates shortening and tightness in these muscles. Then there are normally significant muscular imbalances in the pairs of muscles which should theoretically be evenly balanced to work efficiently. All our slumping and rounding means that the opposing muscles which should be pulling our shoulders back and our scapulae down and in are then weaker. This in turn makes it even harder to fight the pull of the short, tight, strong muscles that are responsible for rounding the shoulders forward and pulling the rib cage up.

When you add repetitive activity such as sports, work and just general day to day moving, this tends to exacerbate both underlying structural or genetic issues and the postural issues. It becomes a chronic, self perpetuating problem. The longer this imbalance goes on, the harder it becomes to fix. Stretching, trigger point release, exercises and physio can sometimes only do so much with muscles that have been shaped wrongly over years or decades. Sometimes surgical intervention then becomes desirable or necessary to try and undo the secondary consequences like the pressure on nerves or blood vessels causing pain, loss of function and so on.

Does that help explain a bit?