[ivalm]

Asset values can just increase. Alice who has $10 and 0 units buys 10 units from Bob who has 10 units and $0 dollars. Alice then sell back to Bob 5 units for $10 dollars. Alice now has 5 units (worth $10) and $10, Bob now has 5 units (worth $10). Total wealth in the system went from $20 to $30.

In addition, companies produce things, some of that wealth gets returned to the investors through dividends, interest (eg on bonds) and buy backs (in my example, let's say each unit generates $1 in dividend, now total wealth is $40(!) while starting at $20, including $20 cash (starting from $10) and $20 worth of units (starting from $10)).

In fact, we see this growth everywhere around us as both the amount of people and the amount of goods and services per person is increasing!