Because male-male USB plugs still have their power-providing contacts out of reach of your grubby little fingers AND only deliver 5V DC.

A male-male wall plug has the power-providing contacts exposed (while "hot"), delivers 120V A/C, and it would require redesigning nearly every single appliance that uses 120V A/C in order to make it worthwhile. Not to mention that 1) they're explicitly illegal in the U.S. and 2) they violate nearly all existing electrical codes.

Are you trying to electrocute yourself?