The problem with head banging is that it can cause much internal damage to your skull. For example, if I break my ankle it will show on an xray and it can be casted and heal. But if I just bruise it, it will not show on an xray. An MRI is needed for that and it takes much longer to heal.

You have probably heard of shaken baby syndrome where a baby is shook and their skull and brain hit and extensive damage can be done.

You can force yourself to binge and purge, swallow glass or other sharp things, or otherwise cause serious injury that cannot be seen. Just because you cannot see the bleeding does not mean it does not exist. And it is much harder to take care of internal bleeding.
If I cut myself, I can stop the bleeding, clean the wound, bandage it and keep it protected until it heals. If I force myself to repeatedly vomit, I can rip my stomach lining, cause internal bleeding, or even rupture my esophagus and I may not have any idea this is happening. And even if I were to start throwing up blood, I cannot fix that on my own. I would have to go to the ER.

Now, the argument could be made that hitting (thighs or arms) instead of cutting or burning is safer. Again this is debatable, but at least with thighs and arms you have a lot of fat and tissue to absorb the impact and you are not putting your brain at risk. I hope that this was not upsetting or graphic. I know everyone here is just trying to help people be safer.