It isn't hard to think of reasons for this, but many of of those reasons invoke the idea of prospective kin - unborn children.
Most children have a probability of becoming an ancestor - and it is possible to extend this idea to children that haven't been born - and to children that haven't been even conceived.
Pretty standard kin selection models can thus be applied to courtship behaviour, bower construction, nuptual gifts - and so forth. These are cases where cooperative behaviour without relatedness occurs. In fact, there is relatedness - relatedness to unborn children.