Saving The Commission
Most "for sale by owners" attempt selling their own home for the simple reason that they feel that they can sell their home themselves and save the real estate commission.  Sometimes home owners think agents are just putting a sign in the yard and an ad in the paper to sell a property and that seems easy enough to do, so why pay a Realtor.
If a home owner knows exactly what their home is worth and is willing to wait it out for the time it takes for that "right" buyer to come along, then it is possible.
In most cases, where the home owner was successful in selling their own home, the price obtained  was lower on average than the price brought by a Realtor.  This price difference has shown to be more than the commission saved, showing that the homeowner actually lost money by selling themselves.
Part of the problem is that the buyer sees that there is not a Realtor involved and also starts to subtract the commission from the price.  If the buyer saves the commission, then the seller doesn't.  Sometimes the buyer has been out looking at other homes with a Realtor and has a better idea of what the home is worth more than the seller.  From the buyer's point of view, they can buy any house through a Realtor and get the Realtor's services at no cost.  Why should they buy direct from an owner and pay for something they don't get.
Another problem is that "by owner" sellers get frustrated and tired after putting in so much time and work trying to sell their home and become more eager to accept an offer from a buyer that comes along.