Virtually grounded negative input op amp exists but no virtually grounded positive input. why?
It's negative feedback and high open loop gain that creates a virtual ground. You don't get negative feedback to the non inverting input of any differential amplifier; you get positive feedback and this does not create a virtual ground.

You can add an inverting amplifier after a differential amplifier and this, in effect, swaps the pin which will become a virtual ground when negative feedback is applied.