Because they are desperate to do business and think you will pay more than they are willing to take.
I quote a price and that's it. Hate it when someone reduces it all to the level of a bazaar.
I say 100, he says 50,
I say 100 he says 55,
I say 100 he says I'm not playing the game and offers 60.
I tell him to go jump in a lake, I thought this was business not a game.
Negotiating is a little more intense and complicated than that.