It seems like a lot of the excess value will be captured by the person who attended college, if that's true aren't they the very person to pay for it? If it isn't why bother with it at all?
Done right, they produce more wealth and some of that wealth goes to society. It's not just that they pay more tax; they also simply produce more wealth, which is not a zero sum game. Over a lifetime, society benefits and the person benefits. Everyone wins. Done right.
Stretching the discussion a little, we could ask the same question about all education. Why is any of it free? But the benefits to the United States (for example) of almost everyone being able to read and write (for example) are incredible. If a large proportion of the US population had dropped out of school before being able to read and write (because of the cost), the US would be extraordinarily less wealthy. Everyone would lose out.
This isn't the kind of value they could use to build a big castle and cut themselves off from society. Education isn't like giving someone money - they still have to go out and use that education to make money. It's the society at large that reaps benefits of an educated person - through more specialized work being done, and through better social interactions.
