Why does my cat bite herself pulling out fur leaving open wound always in same place.?

It could be any number of problems, including behavioral, allergies (to food or environment), mange, or any wide variety of things. You should definitely take the cat to the vet for a better idea on what it could be, and I would suggest trying to switch her to a different food, preferably a good, grain-free diet. I recommend Blue [Buffalo] Wilderness, Orijen, Acana, and Wellness CORE. If different food does not help the situation, then it is probably otherwise (but I would still recommend keeping her on a good diet, because cats are obligate carnivores, meaning they need meat, and many of the cat foods out there are filled with too many grains or vegetables).

It seems like she has a hotspot. She doesn't make the wound on purpose, if it is a hotspot, she's been scratching at it, and it probably hurts by now, so she's trying to clean the area up. Shave around the area gently, and put some Polysporin on the wound. Also see that she doesn't bite at it anymore.

Hopefully this helps :)

she has a skin irritation possibly an infection or mange or a tick, if you are feeding her dry or pellet food that is a big cause of skin problems in cats and also kidney failure, strange how the food vets sell also creates secondary income treating the consequences

She might have mange :/

she could have ocd