Well, the "Bay Area" encompasses about seven or eight counties, and there are WalMart and Sam's Club stores in some of those counties. However, there are none in the East Bay, Marin County, and San Francisco, so that's four counties. Well, there maybe a WalMart in the far eastern area of the East Bay, but none that I know of. The general area in which I live, Marin County, and San Francisco, aren't ideal for WalMart stores anyway - not much space, very high land prices, and the demographic here is not particulary WalMart oriented.
I have to confess having shopped in a WalMart - or maybe it was a Sam's Club - I was somewhere in Missouri and stopped in to one of the stores because I needed a quart of oil, and the store was handy. First and only time I crossed the portal