Because there is no icon for it in any of the maps.
https://wiki.openstreetmap.org/wiki/Tag:emergency%3Dlife_ring
https://wiki.openstreetmap.org/wiki/FR:Tag:emergency%3Dlife_ring
But of course it is good to map them as it might show up in the map at some point in the future and there might be applications that use the data even when it is not shown on the standard maps (Like AEDs as an example https://wiki.openstreetmap.org/wiki/Tag:emergency%3Ddefibrillator)

Actually you should not add amenity=life_ring to it as this is an old way of tagging it that was replaced by emergency=life_ring
https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dlife_ring
And I guess that even the name tag is kind of wrong as this should only be the name if it exists (which I do not think here) not a description.
