Why is a rectangle a trapezoid?

A rectangle is not a trapezoid. A trapezoid is defined as a quadrilateral with one pair of parallel sides and another pair that is not parallel. That description cannot be applied to a rectangle.

It is only if you define a trapezoid as a quadrilateral with one pair of parallel sides and say nothing at all about the other sides, can a rectangle be said to be a trapezoid. But you would have to be mathematically incompetent to use that as a definition of a trapezoid.