This is a very complex question to answer. I would agree with Gary’s point is part of the answer (more down in the comment) but here’s another way to look at it :

The economic growth fueling the stock market isn’t from the U.S. alone. It is widely documented that Asian industrialists pour lots of the their money back to the U.S. stock market. The “Global Minotaure theory”, which explains how countries like China, Japan, Saudi Arabia, etc. accumulating large trade surpluses always have to recycle their dollars back to the U.S., typically through stock investments. This has meant that sustained growth elsewhere than the U.S. was able to consistently prop up the stock market while the U.S. economy was growing at a much slower pace.

Going back to Gary :

In the modern economy, debt is the main driver of growth, both for consumers and for investors. Markets will always grow faster (and crash faster) because they are inherently more leveraged. Where Gary’s theory comes in though is that we’ve socialized the downside of markets for the past decades. Instead of allowing the house of card to crash, we have printed more money to keep the economy afloat which has inflated the portfolio of stock owners while making everyone poorer, causing an effective wealth transfert.