According to the data I found online, it appears that the density of CORS (Continuously Operating Reference Stations) in the US is significantly higher compared to other countries. Why?
This seems to be an issue of data aggregation - none of these maps show all of the CORS stations in each region.
The NOAA coverage map is composed of the stations provided by all state's Dept. of Transportation, several federal agencies (coast guard, NGS, FAA), and some, but not all University operated stations https://www.ngs.noaa.gov/CORS_Map/ (select CORS Agency Map under "Choose Map")
The EUREF map is composed of stations that are a part of it's network, but not all stations in the area. Look at UK stations here, for example (https://www.ordnancesurvey.co.uk/gps/os-net-rinex-data/) a much higher density than in the EUREF map.
The Australian map also does not seem to have all stations in country listed. Here is a map with far higher sensor density (http://smartnetaus.com/home/index.php?q=map)
Ultimately this seems to come down to an issue of data aggregation. The NOAA has better data aggregation than the other maps, but not of them show all CORS stations, so we can't make a judgement based on this.