The reason that it's not done that way is three-fold:

(a) People's contributions are not enough to cover their social security. Instead, they depend on the current workers paying in more than the previous generation did (in a perpetual cycle).

(b) Liberals oppose the idea of personal responsibility. If you don't save any money for retirement, they think society still has an obligation to fund your retirement.

(c) It's a benefit which goes to old people and old people vote a lot.

In reality, it's just a cleverly marketed way of stealing money from the youth and giving it to old voters.