Humans use other clues to estimate depth, i.e. previous knowledge of the objects. However, strictly geometrically speaking, you cannot obtain depth from one single point of view.
Think of it in simple terms, suppose you are in a dark room and there's your tv led on:
Now, imagine you have all the camera parameters (focal length, distortion, resolution), how far is the tv?
I bet you cannot say! Although some estimation algorithm exist (that use previous knowledge, like humans do), triangulation is necessary for computer vision.