They squeak because they get contaminated with something. For example, wax on the windshield from running the car through the car wash will cause them to squeak. In that case, cleaning the winshield and the blades of all contaminants with window cleaner may get the blades to stop squeaking. Sometimes, they are too far gone.
The blades might need to be replaced
They squeak partly because the rubber blade has gotten brittle with age. The other reason they squeak is also because of age: The wiper part is no longer flexible and wipes across the glass at an angle.
They squek some times because they don't have anything left on them that dosent make them squek. Hope this helps