"Mostly only cheap crap items do this."
Such as the Tektronix 211, 212 and 214 handheld oscilloscopes of the 1970s and 80s, anywhere from $1800 to $2500. These are called "reactive dividers" and Tek used two hefty Mylar capacitors in series for the divider, used both to power the little scopes and to charge the internal battery packs.
The reactive divider (if fully capacitive) does offer line isolation, but that isolation is limited. If the cap tied to the line shorts, the world literally goes up in smoke. The little 200-series scopes were double-insulated, so the isolation problem was a moot point. A reactive divider has little actual power dissipation as compared to a resistive divider. In fact, the divider you mentioned with one cap and one resistor will dissipate power through the resistor, so isn't as economical.
Dean