Weakness of conviction and sadness at a real loss.

If, when I was, say, 12 years old, my Dad had said he was going away for fifty years but he would return when I turned 62, I would have been devastated... but looking forward to seeing him again anyway. Time apart from loved ones is always painful. Even for adults. If I believed in an afterlife today (which I don't) and my closest friend or family member died, I would still grieve the forced, total loss of communication with that person, even if I was confident it would be restored after my death.

It's a tragic experience to loose a loved one. No matter what belief system is. The pain of the lose and waiting can be heart breaking for the religious fools. I have accepted there is nothing after death. It was one harder things to accept for me, when I left the dark side. I'm sure it tests the religious fools faith in the bs and maybe it's enough for them to become an atheist and start there journey of enlightenment.

Because they don't actually believe. Deep down inside they know the truth, but they lie about it to themselves and others because that's what they've been trained to do. It's like the story about the Emperor's New Clothes. Everyone knows the truth, but nobody gets to say it.

To be fair, religious people believe in grieving, knowing they'll never see the loved one again in this life. But that said, your question is my question too. I've seen grieving Christian parents' lives ruined when a child of theirs dies at a young age. The hope of the resurrection they claim changes everything seems in fact to change nothing.
Of course, this doesn't go to the validity or otherwise of Christian belief, but it surely does expose what such religious persons actually believe. Actions are louder than words; they grieve no differently from others, possibly worse, because they keep telling themselves it's for the good yet can't emotionally accept that.