Everyone has collectively agreed that the market goes up over time. So people keep putting their money in. Stock prices go up, which generates more money for people. People spend that money, which generates more profits for companies. Which in return drives stock prices up even higher. Which then again generates more money for people, who then spend...
Couple that with governments printing money and people/companies borrowing even more money. The money has to go somewhere, so I imagine a large portion of it just keeps the party going.
It feels like a self fulfilling prophecy at this point. Almost like everyone plays a game of musical chairs to make as much money as possible until something seriously breaks, the market resets, and then we repeat the whole process again for the next 10 years. Rinse and repeat.