I think the point is that Morality can very simply be an evolved human trait due to the massive second and third order effects that derive from it or the lack of it.

Let me put things this way: psychopathy is a condition people are born with, were they are unable to empathise with others so amongst other things they don't feel the hurt of others and are thus capable of inflicting great hurt to others, lie and do all sorts of socially-reprehensible things without feeling a shred of guilt. In practice they will do what's best for themselves with no consideration for others except for the purelly rational "can they punish me if I do this" (in simplifying all this a bit since psychopathy is actually a range rather than simply an Yes/No thing).

Anyways, around 3% of people are born high in the psychopathy spectrum. Now, if psychopathy is "doing what's best for yourself with no consideration for others, no guilt, no conscience to be weighted on" - on other words, no moral, just limitless personal upside maximization - which one would expect is the best possible survival and reproductive strategy there is, why doesn't human evolution lead to 100% of people being born psychopaths?

My theory is that societies with too many psychopaths collapse, removing the psychopaths from the genetic pool, plus psychopaths have trouble cooperating (for the obvious reason they only care for themselves) and thus can't survive the kind of danger that can only be defeated by a group of people.

How would that be. Well, they're pure takers - why tire oneself by making if you can get away more easilly with taking - and they're not good at cooperation (both because they only care about themselves and because when other people spot their character, they don't trust them and don't want to cooperate with them), so any society with too many psychopaths is less productive, has less resources available (too many takers too few makers), it stops evolving, can't properly organise a collective defense system and eventually gets overwhelmed by some other society without such problems.

In other words, even whilst the 1st order effects of being entirelly amoral and purelly out for yourself are pretty positive for that individual, the 2nd order effects (such as others tending to shun that individual) and 3rd order (societies with too many such people end up collapsing or conquered) make being amoral a non viable strategy, except if they're a small fraction and most people around are moral.

This last part is just my theory for why, but certainly the part that only 3% of people are born like that is a pretty good indication that for whatever reasons an amoral behaviour in humans is not a winning evolutionary strategy even though some might think at first sight that it would be.