Actually they did, in USB 1.1 spec. 6.5.1: "The USB Icon is embossed, in a recessed area, on the topside of the USB plug. This provides easy user recognition and facilitates alignment during the mating process."
That might be easy to figure out on a laptop but on an AC-charger? A workstation? A lot of ports are sideways.
And, the ports on my monitor are, by any conceivable scenario, rotated 180 degrees from what they should be (they are on the side and "up" in that scenario is facing away from me, towards the wall).
Stuff like this requires consistency so that you can trust your instinct, USB have failed miserably in this regard.
And the Apple dockplug is even worse IMHO; besides looking near-identical on both sides, the cable with my iPad 3 printed the logo so lightly it's quicker to guess vs. trying to find it.