Because our writing system is unidirectional. Documents have a direction that they flow in (typically vertical), and text within those documents is written alon

Because our writing system is unidirectional. Documents have a direction that they flow in (typically vertical), and text within those documents is written along the other axis (typically horizontal) and wraps when it hits the edge. So in a viewport, a document has three fixed edges defined by the viewport dimensions (typically top, left, and right), with the other edge (bottom) defined by the size of the content. So the horizontal axis works in a very different way to the vertical axis.