Social Security was passed in the New Deal under FDR as a tax. Because it is a tax, the government is not obligated to pay it back to you. They only promise to do so. Since SS is a tax, the government spends your retirement on whatever they government needs money for at the moment. You do not get the same money you paid in. The government does not have enough money to pay everyone back because they spend it, it is not saved. Why would anyone trust the government more than themselves in setting up a retirement fund?
Yes, privatization of social security improves individual liberty, because the system provides much more satisfaction to the individual saving. The person saving can save to meet their goals. Returns are much higher in the private sector. Those paying into the system will be a lot happier seeing their money earn large returns.
Social Security is and always should be a government responsibility. This is a program designed to care for the elderly and disabled, and to put that in the hands of a for profit corporation that is only concerned with increasing profits every quarter is dangerous and opens this vulnerable and often disenfranchised group of people to further financial harm.