Here's why:
* Compound nouns: "Square foot" is a compound noun, meaning it's made up of two words that function together as one unit.
* Hyphens are used for:
* Compound adjectives: "A three-foot-tall tree" (describes the tree)
* Potential ambiguity: "The re-creation of the play" (to avoid confusion with "recreation")
Since "square foot" is a well-established compound noun, a hyphen is not needed.