For the same reason that a man is always a person but a person is not always a man.
And the same reason that a 5th grader is always a student but a student is not always a 5th grader.
A square is a special kind of rectangle ... one with all equal sides. If all the sides are not equal, then it's still a rectangle, but it's no longer the 'special' kind of rectangle.
<span>A square is a special kind of rectangle, it is one where all the sides have the same length. Thus every square is a rectangle because it is a quadrilateral with all four angles right angles. But not every rectangle is a square, to be a square its sides must have the same length.</span>
