It's worth considering that no group of people will basically ever say "no" to one of their member doing something that the rest of them benefit from. Individual sacrifice for the benefit of the group will always be welcome, but individual selfishness will only be tolerated if it doesn't harm the group.

So it make sense that cultual memes have evolved to favor selflessness, and often condemn selfishness.

And whether we're conscious of it or not, most people essentially equate morality and our sense of pride with group approval - which is where the myth of selfishness being "immoral" arises.

I've written more about that latter point here: https://medium.com/@nebulasaurus/do-pride-and-shame-inform-or-confound-moral-discourse-80c0b12e4800