More can be written, but you might like to start with the "Sparta" FAQ section (this is on the Ancient Warfare wiki page -- not the "Sparta" section of the Antiquity page, though that also has some discussions on Spartan warriors). The first two discussions, by /u/Iphikrates and /u/astrogator , discuss both the reputation and the reality.

This is not to discourage discussion. More questions, data, and debate are welcome.

As /u/scarlet_sage noted, I've answered this exact question here. The short of it is that they are depicted that way because of what's known as the "Spartan mirage" - the mythologised version of Sparta that reflects what later authors wanted the Spartans to be. While they actually did enjoy a brief period of military dominance in mainland Greece, this was not for the reasons it's popularly thought to be, and the Spartans certainly never were superhuman warriors.

For more reading, have a look at the section of my flair profile where I've gathered my best answers on Sparta.