Because mega-rod and giga-rod sound hilarious to me.

Also those units end up with a lot of pretty alien things as their bases, 10e-35 meters isn't super useful for human things. Whereas a rod maps pretty well to "down the hallway a bit", and a half a rod works pretty well for "the other side of the room". meter and foot work well for this as well, but they don't sound nearly as cool, though I would be willing to measure distances in "kilo-feet" just to screw with people. Alternatively a system of esoteric units starting with the meter would also work for what I'm thinking about (say, 12 meters to the dimber, with 60 dimbers to a schlimmel).