The wipers glide on a molecular film of water.  They skip when they
encounter a surface that doesn't have this very thin film.  Rain-X IIRC is
an alcohol based material that prevents that film from occurring and doesn't
allow water to adhere to the glass.

Wipers skip on the dry areas that the Rain-x produces.

Just for fun, surreptitiously treat a friends driver side only.  His
passengers will be terrified in the rain.