I used to inspect food facilities. One of the things we did, for training purposes only, was to do cultures on people's fingertips. We'd grow out all kinds of stuff from folks who claimed to have just washed their hands.

One of the main reasons, I believe, is a thing I see women doing in rest rooms: they stick their hands under the water for a few seconds and dry. I suppose they think it looks proper, but they're just giving the little buggers a drink - it's the soap that kills them and makes them let loose of your hands and gurgle down the drain. Not so bad as long as they aren't making your dinner because there likely isn't enough bacteria in a handshake to make you sick. They're probably more of a danger to themselves and their families by carrying viruses they got from the handles, doorknobs, counters, telephones, the coffee pot and (in the case of 'rinsers') faucet controls.