It's because they're big companies. The bigger they get, the less they can focus on any one thing. More people means more risk of someone being unqualified, and less oversight through more layers of middle management, most of whom are also unqualified.

Your local business employs fifteen people. One owner, two managers, and 12 staff. Everyone knows everyone and if they're truly bad at their job they can't deflect and skate (unless the owner allows it).