Possibly due to point 2, for instance.
> What kind of reciprocity exists if we talk about healthy adults taking care of disabled children that would likely die in few years?
You do realize that people may become disabled in their lives, right? It's not just children.
In any case, that can also be explained by point 3.
> Oh, how close it is to saying "people are often being selfless"!
But it isn't, and that's the point. This is a case of self-actualization, the highest expression of the "self". Some may argue that this is "selflessness", but I argue that this is "selfishness" in its purest form: the pursuance of one's highly personal goal, i.e. psychic profit seeking.
Eventually we benefit from it in our old age and teach our grandkids to remember about it as they get older.
How these social pressures could arise if all people in society acts out of selfishness?
> 2. Setting examples for reciprocity
What kind of reciprocity exists if we talk about healthy adults taking care of disabled children that would likely die in few years?
> Some just see value in helping others, either in the act or the results.
Oh, how close it is to saying "people are often being selfless"!