My guess would be that HTML and CSS originally were meant to deal with sites that resemble a paper document (i.e. a collection of A4 papers) so it tries to be consistent with that:

- the maximum width of a paper document is simply the width of a piece of A4 => so an element can have a well defined width, that being a percentage of the width of the document

- the maximum height of a paper document though.. doesn't exist as you can always add more papers, similarly css can't define the height as a certain percent of the document, so it chose to look at it's own child elements.