NobodyReally
Superstar
You need to use the stones in conjunction with a tool/contraption/vessel. You can't just hold it in your hand and do shyt with it. Besides the end of GotG no one has used a stone with their bare hands.
Who said he can use it? Remember what happened when Red Skull tried to use it?
OK, that makes more sense.
But Loki has used the tesseract before, yes? Is that why his sphere thing was so important?