Find me 7% growth on $10k dollars for multiple decades running and you can have all my money. In other words, that's a very unreasonable mean growth rate.
Here’s what you’re missing. Imagine you retired in 2007.
If SS was invested in private stocks you’re likely destitute and possibly dead, because your home, 401ks and social security are all underwater.
Social security has less volatility and has an associated cost for that.
Am I missing something here?