I cannot be sure, but it seems obvious to me that this is a deliberate and somewhat arbitrary choice by the programmers of the renderer. It should be considered a secondary point, as is all of the renderer, and the maps it produces: the primary goal of OSM is to create and maintain a database of geographical information.
Also, the database contains such a wealth of detail, in places, that to render them all would make the map fully unreadable; so choices had to be made. I feel sure there must be lots of discussion on what to render and what not; and there will and can be no set of choices that satisfies everybody.
