I don’t think that at this time in 2020, we know the complete answer to this question. 
In biological terms no other mammal seems to self-harm, in the same way humans do. It seems there are complex neurological pathways in the brain that are set up when the cutting occurs, which are quite addictive, so that the cutting behaviour is likely to recur. For example we know from people who have tattoos that the actual tattooing process causes pain, but the person goes back for more ‘ink’. In the same way cutting is addictive it is hard to stop.
There are many psychological hypotheses as to why people cut. Do they do it as some sort of punishment, some relief of internal tension? Many commentators feel that in some way the cutting is a response to previous trauma. Prof. Van der Kolk in the USA believes the body ‘carries’ the memory of the previous trauma. Whatever the hypothesis, each person is unique and the reasons for their self-harm need to be examined within the context of a safe therapeutic relationship. It must be remembered that some people cut and have no memory of doing this until they see the marks and the blood on their bodies. They are not aware of their emotions at the time of the cutting. They are dissociated.