Why do you say 'God bless you' and not 'God blesses you'?

I thought the correct sentence should be God blesses you,God blesses us,She blesses you,God blesses us.But why many people says that God bless you,She bless you,God bless us?What i knew was anything singular subject/singular nous,the verb should add 's' at the bak of the word.But this case i cannot understand why no 's' is added on the back of the word bless