Why? The shaft adapter has a double flat ground on it.
The problem is it is a odd size tool wise. It is 29/32" open end wrench size and a 15/16" (24mm) is a loose fit and 7/8" won't fit. The 15/16" will work most times but I prefer take an el cheapo wrench and grind or file the opening to 29/32". Otherwords you need a custom open end wrench for a good fit. Or a large adjustable wrench would be a second option. Most mechanics do have adjustable metric/SAE wrenches.
But for piece of mind don't ask the Lowe's tool guy for a Metric adjustable wrench. The one I did ran around like a chicken with its head cut off; until, he asked his supervisor. Then I got some really hard looks. But I bet he will remember that one. The handle length can spec'd in mm.
Why Hydro Gear used an odd ball size is just to be a pain I reckon.