If you have painted something with oil based paint why can't you clean the brushes with water?

Because soap and water are NOT oil based. Oil based means you need to use an oil-based thinner to clean brushes and spills.

The materials used in oil-based paints are not soluble in water, even with the added surfactant effect of soaps. (Some paint may come off hands as the soap loosens the dead outer layer of the skin.)