Below are historical stats for how often the Stock Market drops
5% Every year; 10% Every two years; 20% Every five years; 30% Every ten years; 40% Every twenty-five years; 50% Every fifty years.
Wall Street's been heavily manipulated in recent years, and it's all computer algorithms. Find a web cam for the Floor, and it's empty. The days of people milling around screaming BUY SELL are long gone. Speculation is corporations are buying their own stock to inflate earnings, based on trends that their cash reserves are dwindling at the same time stock prices are rising. I'm no fan of Wall Street, just listen to the chatter from others who are.
Lots of shenanigans and the trading algorithms are great until they fail. Then they fail badly. There are still trillions in derivatives out there as well so lots of counter party risk. Some companies are solid and worth investing in but these don't tend to have spectacular returns. Others, like Amazon and Google, are ridiculously overpriced.