Why? Well because you're too stupid to manage for yourself, and the Social Security System funds are out of their reach. So you need to give money to the state to manage for you. This would be in addition to the Social Security Ponzi scheme, which doesn't actually work in your favor. This is the same state that has a massively underfunded state pension system by the way.
No - Really. Why?
Their bankrupt public sector pensions, woefully underfunded due to the need to buy votes. Access to private sector funds would help pay that off - how they're going to pay off the private sector once they've stolen the funds? Well, they'll just get the tax payers to bail them out. Yep, you'll get put put in 3-5% and later you'll get to pay more taxes to pay for that money they took from you earlier and gave to state employees.
So if you think opting out of this - which employees can do (at the moment) but employer can't, get over it. They're going to take your money one way or another.
Do yourself a favor - if your a conservative get out, get out while you can still afford it. If, on the other hand, your a democrat or progressive - please stay there. I'm not sure we can stand another wave of the California Liberal Plague.