Right, the orthodox economist's model doesn't apply to reality. That's why the model is implicitly a prescription or an ideal rather than a scientific description of facts. Implicitly, the idea is that we're supposed to approximate that ideal, to make real-world economies work more like the utopian ones that economists imagine. The more selfish we become, the happier we'll be because capitalism will satisfy all our desires. Of course, this makes orthodox economics more like theology than science, but there you go.

And I'm talking about the dominant regime in economics, not about the heterodox literature. So those links are red herrings.

The point isn't that economists think altruism is impossible. It's that their predominant model of capitalism is a prescription of an ideal system in which the participants are selfish because capitalism feeds more on selfishness than on altruism.

What would a capitalistic economy look like in which everyone refuses to put their interest ahead of anyone else's? Businesses would be charities, there would be no private property, and the economy would be a giant commune. So yeah, it's just disingenuous for you to pretend that apologists for capitalism aren't much more interested in facilitating our selfishness than our capacities for altruism.