Why did Americans invest their money in the the stock market?

The Stock Market has generally been a good investment. It goes up and it goes down, but in the long term it goes up. Lots of people have profited from their investments in the stock market, even though sometimes people lose a lot of money if they make a particularly unwise investment. Remember that people who have large amounts of money need to invest it in something. If they just keep wads of currency in their safe, it will gradually lose value due to inflation. Money has to be well invested, just to retain its value.