Why rudder is not fitted forward of ship?

Every vessel has a pivot point or center where it turns the best. The stern of a vessel would be hard to control if the rudder was at the bow. When docking, if the rudder was at the bow, you would never be able to get the stern of the vessel to the dock. Once you got the bow to the dock or pier, there would be no way to control the stern to make it swing. Once the bow is at the dock, with the rudder at the stern, the force of water passing across it from the propeller will push the stern into or away from the dock relative to the angle of the rudder.