Why the concept of ghosts...spirits...zombies...halloween?

There are two schools of "thinking" here. Some people want to believe there is some kind of life after death. But anyone who has stared at a corpse (at funerals...mortuaries...accidents) long enough realizes how impossible the whole scenario is - life is no longer physically possible. It would be completely scary if it jumped back to life.

1. We like to believe in some sort of Life after Death, or Life without Death.
2. But...But...Life after Death is absolutely revolting and physically impossible.
3. Life without Death is also physically impossible. (Mortality)
4. Death cannot happen without Life first.

1 and 2,3,4 are contradictory. We must therefore pick our sides if we are to become stable, and the real side if we are to live as a happy integral whole in the present reality.

To resolve this cognitive dissonance, we create a new part, associated with the feeling of fun (or spookiness, who is more like fun and fright combined). In this way, subconscious desires have another place to manifest themselves, free from the "bounds" of reason and rationality. Here, the desires can roam free, like people in white sheet costumes...

What about ghosts and spirits? And why is fear the predominant emotion culturally associated with them?

Personally, I see this as a fear of the self by a fractured mind. Many movies have used "evil spirits", unseen forces to symbolize internal fears, conflicts.
On a deeper level of the "collective unconscious", It's a natural phenomenon that has existed for as long as the human race. Psychological trauma causes some people to hallucinate, replaying traumatic events of the past. In doing so, they repeat the emotions they actually need to replace in order to move on. This inability to live in the present is clearly related to most ghost stories which deal with the past.

“People are stupid. They will believe a lie because they want to believe it's true, or because they are afraid it might be true.”

And it can only grow deeper.

So stop "celebrating" Halloween and go do something really fun and worth your time... because you only have less than 29,220 Days*, and counting...

"Happy Halloween(2013)!"

 *Assuming you are 20 now and have a life expectancy of 100 years old.