Its all to do with refracted beam patterens and the extra brightness. Dirty light covers, not lenses anymore, can cause refraction that splits the brighter beam casuing issues for oncoming vehicles.
Arguably the washers can't clean of bugs etc so they may not be a total solution anyway. The old Volvo's, Saab's and Merc's fitted with little mini headlight wipers were better at cleaning but this was long before Xenons and they are an issue in crash situation and the 5th star of the Euro Encap test (5th star is all about pedestrian safety).