Making a minimap for all these maps is totally possible, it is just the map maker was too lazy to spend the 5-20 minutes to make one.
The problem is that in the grand scheme of things, in my opinion, it isn't worth it because that means someone would have to decompile the map, fix everything that broke during decompiling, then have to re add it to the server so everyone has to download another ~100mb map that just has the minimap updated. None of this is that challenging, but still takes time.
Now I'm not saying minimaps are bad because they are extremely useful and wish the map makers weren't lazy. However, I think it would be better to wait until a big map update or something in order to update the minimap.