Because if you only paid for the roads that you did drive on, there would be a tollbooth every hundred feet and the entire thing would be enourmously impractical and frustrating? There's a reason people call libertarians selfish and unconcerned for anyone else, and that's because it echoes in your every statement. Who cares about the unemployed as long as I have a job? Who cares about a social safety net as long as I have rich parents? Who cares about health care as long as I don't currently have any health problems? And on, ad infinitum. As a philosophy, as a political ideology, it's self-centered and short-sighted to the point of absurdity. It treats capitalism as some kind of benevolent god that can do no wrong, and when it does do wrong, blame is shifted onto governments, victims, and anyone else standing in the crossfire.

Here, have a quote from Eugene Debs:

"Now my friends, I am opposed to the system of society in which we live today, not because I lack the natural equipment to do for myself but because I am not satisfied to make myself comfortable knowing that there are thousands of my fellow men who suffer for the barest necessities of life. We were taught under the old ethic that man's business on this earth was to look out for himself. That was the ethic of the jungle; the ethic of the wild beast. Take care of yourself, no matter what may become of your fellow man. Thousands of years ago the question was asked; "Am I my brother's keeper?" That question has never yet been answered in a way that is satisfactory to civilized society.

Yes, I am my brother's keeper. I am under a moral obligation to him that is inspired, not by any maudlin sentimentality but by the higher duty I owe myself. What would you think me if I were capable of seating myself at a table and gorging myself with food and saw about me the children of my fellow beings starving to death."