In your example Richardson and Nelson get off 16 and 17 shots, while Arenas gets off 9 in limited minutes. Turkaloo gets off 8. Thats spreading it around.
In BB it would look more like Richardson gets off 25 shots, Nelson 9, Arenas 2 and Turkaloo 6.
As others have stated it could be that the SG is typically teams best shooter. And those same guys tend to have less than average handles and/or passing. Also I will add that off flows tend to be lower than the rest of the stats which can add to the SG getting stuck with the ball late in shot clocks, and/or they just cant pass the ball or drive so they do what they do best. Jack a ton of shots lol.
Of course its all just speculation :)