Why can't I buy cashews in the shell, only as mixed nuts?

Several things first.  First, if you look hard, you should be able to find cans of just cashews.  Second, cashews aren't real nuts, same for peanuts; actually most of the 'nuts' we ear are not really nuts, such as almonds, Brazil nuts, pistachios, macadamias, and coconuts.  Third, cashews are not like water, they are bad eggs or nuts, but not real nuts; cashews are bad and they will kill you.  Well, maybe not kill you, but it could give you a mighty rash. 

You see, the cashew fruit contains a caustic phenolic resin, urushio.   This is the same irritating material found in poison oak, poison ivy, and mothers-in-law.  The seeds need to be removed carefully to avoid exposure to this vile substance.  This is referred to as habilitating the cashew.  Originally it was called rehabilitating, but the botanists decided it couldn't be rehabilitated if it had never been habilitated in the first place.

So, next time you pop open a can of mixed nuts to pick out just the cashews, give a moments pause to thank all the little people, mostly 8-12 yr. old boys and girls working 16 hour days for unlivable wages who live in India, Vietnam, or possibly Brazil who have gone through the trouble of getting that delicious cashew ready for your mouth.  Unless they are processed by machine or older people or trained monkeys or black magic, in which case I would recommend eating the whole can in one sitting behind a locked door.  Seriously, what good has sharing ever done you?