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!
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!