There is a cost to voting on Reddit, it's the time/attention cost of moving your mouse over and clicking a tiny button. Sometimes you even have to log in! For people browsing a web site, time & attention is a significant currency.

It's true that karma is a flawed as an economic system or a measure of contribution. But, who cares if the numbers are theoretically unsound? The end goal is to generate usage, something which Reddit has been extremely successful at.