This is because we have only 2 ears, and the brain relies on other sound cues to generate a relative position.
You can now tell only that it is to your left or right, you have lost the ability to hear relative position. NOW, turn yourserlf 90 degrees left or right, so the 2 front speakers are directly to 1 side and the 2 rear speakers are to the other. Now, facing directly forward, it sounds wonderfully surround and positional and whatnot. Play a sound source through a 4 or 5 speaker setup. People are out there talking about 7.1 setups are optimal (and you thought 5.1 was a lot!). Technically, in audiophile/videophile circles, there's word circulating that a 4-speaker setup is *inadequate* to deliver a true positional audio.
