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Given that Web Mercator is the defacto standard for web mapping and all basemaps (Google, ESRI, Mapbox) are based on this coordinate system..

Why do many organizations e.g. NOAA not publish their WMS with support for Web Mercator ?

For example, this WMS https://idpgis.ncep.noaa.gov/arcgis/services/NWS_Forecasts_Guidance_Warnings/watch_warn_adv/MapServer/WMSServer?request=GetCapabilities&service=WMS

only supports EPSG 4326 (WGS84) and 4269 (NAD83).

To get this service to overly with a Web Mercator basemap, the service needs to be re-projected on-the-fly, something that only seems possible with desktop GIS tools.

My understanding is that adding support for additional coordinate systems is simply a matter of adding them to the list when the service is published.

Is this really such a burden/consumer of resources to do ?