Why? If a boat is realistically priced then more people are likely to be interested and there is a greater chance of a sale, just less chance of the buyer being able to knock a lot off the price - which is fair enough. Most buyers aren't stupid, they know the difference between something that is realistically priced and something that isn't.
I sell the odd boat myself and if someone wanted a silly amount of money for it, I wouldnt want to sell it. I have to pay to advertise the boat and if it doesn't sell I am left with a bill for advertising, I've spent time and effort advertising, showing people round the boat, its been on our moorings, etc. It's in my interest to have a reasonably priced boat on my books. I suppose for the large brokerages this isnt an issue and they can afford to have boats sticking around hoping someone comes along with more money than sense.