With time and proper care they do heal.
You should probably take your cat to the vet, just to make sure there isn't something serious causing the damage.
The reason it doesn't seem to be healing is because every time your cat steps down, the foot pad flexes and stretches and takes the weight of the animal. This constant movement and pressure are enough to knock the scab loose over time.
What you should probably do is wash the wound out gently with a mild soap and warm water. Then apply some neosporin (the kind without the pain reliever) and then loosely bandage the paw.
If it were me I would change the bandage once every 2-3 days, and give it another cleaning using peroxide. If the bandage falls off in between that time, repeat the disinfecting process, using peroxide and reapplying neosporin then bandaging.
Be sure the bandage is not so tight that it cuts off circulation, but on good enough so that it (hopefully) doesn't fall off on it's own. The bandage is very important because it will keep your cat from licking the paw. From what I have learned, constant licking will only inhibit healing and make the process even longer.
Remember, that the people here cannot replace proper vetrinary care. And since we cannot see or examine the wound, I urge you to take your cat to the vets to determine a proper course of treatment.