The U.S. markets reflect the U.S. ascending to become the dominant world power over the course of a century. Naturally, the markets grew during this period of tremendous economic growth. The investment industry likes to extrapolate from this, and declare it a rule of the markets as if it's a law of physics.

If growth slows or stalls, eventually the markets will catch up. A market that grows independently of the underlying economy doesn't make sense in the long term.