There are many slightly different ways to calculate this. As perceived risks dropped, stocks soared. Everyone buys because everyone else is buying. Well, there's a case for less risk, starting with the decline of inflation. High inflation in the 1970s and early 1980s hurt profits and stock prices in two ways.
Competition compels companies to hold down prices and raise efficiency.
But these forces can also magnify risk. By its nature, new technology creates uncertainty. Profits are not ensured. No one knows what will work. Older companies may suffer from new competition.