Mostly because regardless of price, it seems there's a selection of buyers who simply haggle because they simply would never pay the advertised price, or thats what they feel they have to do, so many people up the price to cope with the hagglers.
Then you have the desperate sellers who sell cheaply because of financial difficulties, and then what you get is buyers pointing to those desperate sales and saying " but that seller sold the same item for blah blah " This is not regular, i should add, but it happens.
I had someone refuse to buy something for £22 because i wouldnt knock £2 off. I simply cannot be bothered with such buyers. I sold to someone else in the end.
If it had been a £100 item, yes would have haggled, but £22 - no