That may be true whirlygig, but probably not the most effective way of market research for the seller ..
In most cases you can see the statement for more expensive or rare aircraft (which more or less supports your view).
However I have a feeling that the seller in many cases doesn't want to "scare off" a potential buyer with a number that looks high, due to the aircraft being in excelent condition compared to most on the market, and hopes to attract the buyer by keeping the number invisible for the time being so that the buyer can be engaged in a meaningful dialogue about the value and the sellers reasons why it is set at that value.
I don't know if this works for anyone else, but the general effect on me is that I just don't bother to call.