Why don't electrical receptacles have more than one ground?
This is because removing a device isn't allowed to interrupt grounding connections

Removing a wiring device from the circuit cannot break grounding connections, or else you are violating NEC 250.148(B):


  (B) Grounding Continuity. The arrangement of grounding
  connections shall be such that the disconnection or the
  removal of a receptacle, luminaire, or other device fed from
  the box does not interfere with or interrupt the grounding
  continuity.


As a result, manufacturers only put one ground screw or terminal on wiring devices, so that you're required to do the right thing and pigtail grounds using a splicing connector of some sort or another (whether it be a wirenut, a crimp, or a push-in type splicing connector).