xhonzi said:

I haven't seen a lot of them (the horror genre seems to be over representin')

Because a great horror film comes up with something really cool and iconic, and some shmuck says, "shucks, that needs one of dem oreejens."

"Solomon Kane" one of my favorite literary characters recently got a 'movie' and the director decided instead of actually doing anything like the many classic works the character was in, he'd completely invent an origin story that's basically the opposite of what the character was, because, in his words, "it's not fair to the audience" to not give an origin.

Here's Kane's origin from the book, paraphrased:

Kane was walking along and found a dying girl.
'The bandit Le Loup did this!' she said and died.
'Men will die for this.' said Kane

It wasn't much, but it was enough.

The following characters became loved by millions without origins.

Conan
Wolverine
Indiana Jones
Han Solo
The Man With No Name/Blondie/Joe/Manco
James Bond
Shane
Leatherface
The Cenobite from "Hellraiser"

Sometimes origins are integral to the character and the story (Spider-Man, Luke Skywalker) but othertimes they are simply unneccesary.