Weak shoulders are the curse of the beginning lifter. For whatever reason, definitely not fate, new and undereducated lifters are drawn to the bench press and curl as their first loves in the gym. Think about it. How many guys do you know that got into lifting by learning squats? Or pull-ups? Or deadlifts?
Finding a beginning lifter obsessed about his squat total is about as rare as finding someone with developed shoulders. Even then, these guys with the best shoulder development are often clearly injecting muscle altering substances. It is exceedingly rare to find a natural lifter with a nice pair of boulder shoulders, supported by equal trap development.
The reason behind this lack of development is because of the shoulder girdle, which includes all three sides of the delts and the traps, is undertrained. Even when it is trained, it is trained incorrectly. Throwing in some half-assed side raises and shrugs at the end of a bench workout is not even close to the amount of stimulation needed for growth.
The shoulders need to be treated like every other muscle group; hit them with heavy compound exercises and then finish them off with some isolation work. There is no other way to build muscle. Training only with compound lifts will never give you the correct mind-muscle connection to equally develop all your muscles, and pumping away endlessly on isolation exercises will only make you better at getting a pump…and maybe jerking off.