Because the sentence focuses on the recent activity itself and its visible result. The activity (painting) is emphasized, and the evidence of that activity (paint all over the floor) is still visible. We don't know whether the painting activity is finished and it doesn't matter. 

Because the room was painted a long time ago and they are talking about a past event. 

Because the action is finished and the result (a freshly painted room) is the main focus. We understand in the sentence that the action is completed, and that's why there is paint all over the floor.