Because it's not all about the money
In The Future of Ideas, Lawrence Lessig states very clearly something important in our society that isn't talked about much these days:
We don't sell the right to vote because the currency--cash--is not the only or most important dimension of value in our society. There are people who devote themselves to careers that don't make them wealthy--schoolteachers and civil servants. We don't think they, by virtue of that choice, should have less power to control how their government is run. They've made choices that result in their having less power in the marketplace; but the marketplace is not a proxy for every domain of social power. As the philosopher Michael Walzer properly observes, there are many spheres of siocial influence in our lives. And we permit power in one sphere to dominate power in another in very few contexts...when the resource becomes foundational to participation in a society, then we assure that it remains in the commons....
I have said this, or something like it, myself before, but I'm sure more people hear it when Lawrence Lessig says it.