Why is this mouse always hard to clean?

Why is this mouse always hard to clean?

I like to use this mouse, but no matter how well I clean the outside cover, it is always gummy and sticky, even though my hands are clean! I use soap, oil or whatever to remove the gummy, sticky feeling on the cover (the black area on both sides of gray area) and still it feels sticky to the touch! I hate that feeling, because nothing keeps it smooth in texture.