Thats because nobody has really given a though to the type of world they live in.
Or they're on something, I mean the people in the future don't feel grief thing makes me think they were on something.
Not to mention how after a situation where billions died Kirk and co. were doing a happy jokey ending.
I do not follow you here. In which episodes did people not feel grief or sadness although they normally should have?
Well that was really a TNG thing actually.