Why do so many people have back and neck pain?

Wear and tear. Many of the low back issues are a result of disc degeneration. Often this is caused by aging, but obesity is a major issue as well.
No back hygiene. Many people don't take care of their back. They become overweight and expect their back to perform as though it is carrying the weight it was designed for. They participate sporadically in sports without proper warm up or stretching.