It's not, and you're acting like that's a good thing. Portability and vesting before an arbitrary service date (like 20 years with a single employer) is a major reason why defined contribution plans are VASTLY superior to old-style pension/defined benefit plans. If Social Security is going to continue to exist, it should be completely transformed into a system where participant contributions are invested into U.S. Treasury debt they own in personal accounts. Even if the accounts are administered by the U.S. government that's infinitely better that the system we have now. The mere idea that you, Eskimospy, and the like are defending the current travesty of a system we have now makes me wonder what your ulterior motives are for not wanting to make such an obvious and hugely positive change.
Defined contribution plans are most certainly not better for employees than defined benefit plans. (they offload risk to the employee as opposed to the employer) This is precisely the reason so many companies have switched to them.
The reasons to not put them into 'accounts' is pretty obvious. What would the account even represent? What happens when the account runs out, etc? Out of curiosity, I'm excited to hear what you believe our sinister motives are. This should be good.