If every renderer rendered every object in the database, the result would be too cluttered to be usable. Renderers only render a selection of features appropriate to their intended use.
The intended use of the default rendering is to aid mappers, and for that purpose it probably concentrates more on things that help the mapper find the correct location well enough to start an editor on them, than on mapping every small object along the way.
Does yours do a better job of fire stations? Mapnik displays what appears to be a Hamat symbol for highly-flammable substances. I look at Mapnik and I see a flammable liquids store. Or maybe it’s something on fire, like an underground coal-mine fire. Maybe it’s an Olympic Flame. When I see that symbol I don’t think fire station.
I ask because I recently added some here: https://www.openstreetmap.org/#map=19/52.08823/-4.64777. I’m still waiting for the allotment association to get back to me clarifying some plot numbers. Which reminds me, the wiki says to put the plot number in a ref but that doesn’t show even in iD (except in the tag list) so I ended up also putting it in a name and then adding area=yes (without the area=yes it’s treated as a way and the name applied to the border). I expect OSM purists will now yell at me for that. However, the guy who proposed it didn’t use ref when he added his own allotment, so I don’t care.
I’m no artist. I can’t even draw the curtains. But a fire engine (what Merkins would call a ladder truck) seems better suited. Almost iconic, in fact. I doubt I could do a reasonable one in a 16x16 image. I’m not sure that anybody could.
I think you’re overestimating the height of the “barrier to entry” for designing icons on OSM-based maps
Whilst “proper design skills” is something that some people creating OSM maps clearly do have, it’s certainly not something that everyone has (I certainly wouldn’t put myself in that group). There was an alternative to the “standard” map called “Osmarender” that was a running joke for many years - it had many advantages (instant update at a time that the standard map updated once a week, rendering some things that the "standard map still doesn’t), but it wasn’t pretty: http://fakestevec.blogspot.co.uk/2012/03/geosuck-0002.html .
If you’ve got an idea for an icon that might be more representative than an existing one it’s still worth sharing it even if it’s rubbish, because it might give someone with better design skills an idea for how to do it and look nice as well.