If you'll excuse the 'analogy', it's a little bit like asking 'Why does a triangle have to have three sides..?' It's defined that way, and, in most music theory (not all, I'll admit; dissent is the spice of academic life, after all...), a chord is defined as being three or more notes heard together. Why does an interval have to be between two notes..? Because that's what an 'interval' is. Why does unison have to have only one note..? It's its definition, that's all. Same with chords, I'd say; may be wrong.