An alternative but similar question would be, ‘Why do cats want to be alone when they die?’. Both questions assume that cats – by which I presume we mean domestic cats – want to be alone when they are dying.
The small wild cats – understanding domestic cats
It is starkly noticeable that in all my reading about the wild cat species I have never encountered a reference to any species wishing to slink away into the undergrowth to die alone. Either it has never been observed and therefore not recorded in a study which is quite possible or the author of the books I have read have found this aspect of cat behaviour unimportant.
I am deliberately referring to the wild cats because if you want to understand domestic cats all you have to do is understand the small wild cats particularly the wild ancestor of the domestic cat, the North African wildcat. Nothing I have read tells me that the North African wildcat wants to die alone.
Hiding when vulnerable
The solitary nature of the North African wildcat means that they automatically die alone anyway, if not in a fight or hunting accident, which makes the question redundant. If domestic cats do run away to die the only possible and common sense reason is that they are looking for cover and protection because they are in a vulnerable situation. If that is true they are not running away to die but running away to find somewhere safe to possibly get better and avoid being attacked.
I’d like to expand on that. Dying cats don’t realise they are dying. They understand that they are in pain and in danger. They can’t see the source of the danger as would normally happen from an attacker. So they can’t lash out at an attacker. The only instinctive alternative is to hide until the pain subsides. It won’t subside because the cat is dying. The cat becomes unconscious and dies but has no realisation that this is occurring.
Self awareness critical?
To be frank people are not sure if cats realise they are dying. It is certainly not obvious. Whether they know they are dying depends on whether they are self aware. To put it another way, cats need to be aware that they exist in the world by objective observation of themselves to be able to realise that they are leaving the world. That’s probably a slightly controversial concept but I believe it unless someone provides a good counter argument to which I am very open.
Why do some people believe that cats go away to die? Do they get the idea from observation and personal experience? What may be happening is that when cats are badly hurt such as when hit by a car they instinctively go under a bush or similar cover. As mentioned this is to find a safe place for self-protection but they just happen to die instead of get better.