Because equity require risk premium over bond. Also because of inflation (I aware of deflation) where things like sales will increase in price and fundamental analysis will take these figure as input to calculate fair value.
Other stuff like oil and gold does not have this property. For example an article was saying oil might drop to 10buck as electric car become the norm.
you are talking about different things. an index is built using public companies, generally weighted by market cap.
equity risk premium is what investors expect to capture.
if you look at a stock like DELL, then the IPO was for $30m, the PE buyback $35bln. Clearly if all companies behave like this then indexes will always rise. If PE companies IPO later in the lifecyle, and buyback in a more distressed state then indexes would constantly fall.