Because they know exactly where everything is (in the place where they last put it) as long as no one comes along and tidies up for them. Or makes them tidy up themselves; once the original where-it's-located connection is broken by tidiness, items will be so well organized they will never be located again.
My workbench is usually fairly messy while I am actively working on project. I have a fairly standard suite of instruments that I have set up on my bench. However, I almost always tear down the rest of the test set up and re-build a new test set up from scratch when I transition to a new board/project. This means putting (horrors!)EVERYTHING including test leads and probes away.
I do this mostly to make sure that I have everything hooked up properly. I have been bit too many times thinking that the set up was correct when in fact, some remnant of the previous project was still connected and screwing up my measurements.
Actually for many engineers "organizing" stuff makes it impossible to find anything. My mess is a highly efficient system, for me alone, were I can locate anything quickly. My wife, who was a superb high-tech project manager and is very highly organized has to put little yellow stickies all over, what most all would consider, our logically organized kitchen so I can find stuff when I cook :)
Perhaps the real test of genius is step B. Step A is to have the messy desk. Step B is to measure how long it takes the desk owner to find an item in that mess. I'd say that's the real test. I have know people that could take a pile like some of those desks and retrieve any item within seconds.
Of course, if intelligence is determined by how fast one can find any given item, then I would likely register an IQ of around negative fifteen.