And every single Bear market has eventually gone back up. Buying shares of a quality stock or mutual fund in a Bear market means you are getting a lot more shares for your dollar invested than if you bought them at market tops. You have to have faith the market will turn around and go back up. Typically Bear markets last between 9 months to 2 years. Be patient and buy through the Bear, then enjoy the Bull.
People do lose money.
They worry about their investment. They are affected by bad news. They sell out during market bottoms instead of market tops. They don't buy when the market is down. The compounding effect of interest on your money over time is incredible.