I think it is because they look old school and filthy. The former is the reason why I buy them, the latter is why I have to wash them more frequently than black keyboards.
Mostly it is just the caps that get filthy, so I remove them and let them soak for a few hours under room temperature water with some dishwasher liquid. A few swipes with a cloth does it for the base.
If I need to clean the insides, I use some canned air spray.
If it is not water resistant, you just pull off all the keys and simply clean it up. If it is water resistant(like Logitech K310), you simply wash it in one piece.