Maybe it is a generous way of greeting to wish more goodness, not only for one night or one day.

I have never thought about that before, but I think myself is right. Turks are very generous people. :)))

I believe it's related to the culture. Plural form is always used in similar expressions. For instance: iyi akşamlar, iyi bayramlar, etc... It is even more than that, mother's day is anneler günü. Father's is babalar günü, teacher's is öğretmenler etc...

Dünyanın en misafirperver halkı Türkler. Türkiye'ye gideceksen, bunu bilmelisin.