Metro icon

There are any number of reasons why the main OSM map style (created with CartoCSS) may not show metro stations as you desire:

1. It is a general map style which has to work not just in dense cities, but in many other places. The style may best be compared with topographic maps, in this case those of IGN, which show details relevant to many different things. (Note IGN only show the metro lines not the stations). The map is emphatically not a city streetmap, although OSM can be used for such purposes with different cartography.
2. The map style must work worldwide and therefore will not use symbology unique to one particular transport operator, or even common to many maps in a given country (or instance the common 'H' in a circle for bus stops in DACH).
3. The map style is very detailed: steps, buildings, shops etc. Large symbols for metro stations would obscure much of the detail.
4. The symbology for transport stops (buses, metro, trams, rail) on the Carto-CSS style has been fairly consistent for nearly 10 years. Whereas it may be unfamiliar to someone used to another map, I don't believe that there is much confusion once one is familiar with the conventions of the OSM map.
5. Particular symbols for metro lines are frequently brands protected by intellectual property law, and subject to licensing and adhering to in-house styles. I'm not sure if this is true for RATP, but it certainly is for the London Underground roundel owned by TfL.

As others have said there are alternative renders (for instance one by MapBox does show London Underground symbols), and you can advocate changes to the CartoCSS stylesheet on github.