When using an aircraft radio to call a Flight Service Station ("FSS"), why do pilots refer to them as "radio?" I understand calling control towers "tower", ground control "ground", Air Route Traffic Control Centers ("ARTCC") "Center", and so on, but why "radio" for a Flight Service Station? Why isn't it "Station?"
"Radio" is just a shortened version of "Aeradio Station" which was the term used before "Flight Service Station".
"Radio" is the default suffix for all ground radio stations; there are many other standard suffixes (e.g. Ground, Tower) that are used instead when they apply, but there's not one for FSS, so they're stuck with the default.
Note that each country can organize their ATSUs how they see fit, and the way the US has split FSS into something separate from ATC is unusual, perhaps even unique, which is why there's not a standard suffix for it.