Social security is an entitlement. They have taken money from your paycheck to fund it. In fact, they have taken more from your paycheck than they will pay back to you in order to pay for an aging population. The extra goes to bonds which the government then uses to reduce inflation when they decide to invade random countries or bail out a bank.

Now, why does the government get to decide when I retire with my own money?

If you don't like it, join the Amish and file a Form 4029.

Well one obvious reason is that you're not retiring with your own money; your contributions fund current retirees.