It's an entitlement because you're not paying for YOUR benefits, you're paying for the current needs of the people who are receiving a check today.
Think of it as taking care of your live in grandma or grandpa. Sure you're taking care of them, but it has nothing to do with what you have in any account when you need/want to be taken care of.
What makes it an entitlement is that when you're of age, the whole rest of the country will be paying for your needs.
the Social Security we have today is not what FDR thought we'd end up with when he signed it into law in 1935.
Life Expectancy:
Life expectancy
at birth in 1930 was indeed only 58 for men and 62 for women, and the retirement age was 65.
Yep, SS benefits were to start at age 65 at a time in history when people didn't live nearly as long as they lived today (20+years longer now).
http://www.ssa.gov/history/lifeexpect.html
The problem is that while SS recipients are now living longer and collecting considerably more benefits, the people who provide the SS cash (the rest of us) hasn't grown at the same pace.