Others have pointed out the need for a rising generation, but I'll be a bit more specific as to the reasons why. We're all working towards (or already in) retirement, and depend on returns from index funds or pensions or rental income or Social Security to support ourselves. All of those depend on the next generation in order to produce returns. Companies won't be profitable without productive workers (and consumers!). Pensions are similarly dependent on market returns. Someone's gotta rent your property, and someone has to pay into Social Security.
So think of the taxes a single person pays as an...investment to help make sure there *is* a generation after us to support us in our retirement! :)
It's also worth pointing out that federal income tax is only one of the many types of taxes we are subject to, and is really the only one that exhibits the type of unfairness that you point out. All the other taxes are more proportional--FICA, gas tax, sales tax, property tax, etc.