Because it's not the only valuable theory in the field?
Rank Theory tethers itself to depressive experiences in situations where social pecking order is the trigger. Basically, to help aide success and survival in a group setting.
Psychic pain hypothesis, for example, comes to a similar conclusion about the evolutionary value of depression, but does so without considering social pecking order.
We know depression can also be triggered just from screwed up biology. So "lack of success" really may not be the issue in a lot of cases.
Rank Theory tethers itself to depressive experiences in situations where social pecking order is the trigger. Basically, to help aide success and survival in a group setting.
Psychic pain hypothesis, for example, comes to a similar conclusion about the evolutionary value of depression, but does so without considering social pecking order.
We know depression can also be triggered just from screwed up biology. So "lack of success" really may not be the issue in a lot of cases.