The article mentions opportunity cost, but punts the issue into the long grass

(or a larger positive effect than any other marginal use to which you could otherwise put those resources, although this latter mode of thinking seems little-used and humanly-unrealistic, for reasons I may post about some other time)

I agree that this mode of thinking gets little explicit use, but I don't think that people can do well in real life without it, so I think that we all tend to bodge up substitutes.

The mode that comes naturally is to undertake courses of action that we anticipate having positive outcomes and to reject courses of action that we anticipate having negative outcomes. We compare against zero.

Sometimes we have a choice of two good options and the uncomfortable realisation that we ought not to divide our efforts between them. Perhaps we recognise a convex situation in which half of each achieves less than all of the lessor. Perhaps it is simply that it is clear than one option is better than the other. If we are comfortable with letting opportunity costs guide our actions we probably get on with the preferred action without commenting on the alternative.

What though if we are uncomfortable with opportunity costs? We feel bad about neglecting to do something positive, and we can assuage this guilt by criticising the second best option by denigrating its merits below zero. This permits us to reject the second best option using our ordinary, lame, comparison against zero.

One cause of the negativity that stops our kind cooperating is that many of us have other things to do and are not comfortable with opportunity costs. Consequently, we cannot just get on with our preferred plan of action but must run down the alternative. We can cure this by becoming more comfortable with opportunity costs.

That doesn't explain people hanging back from collective action on altruistic causes that have come top of their preference list, so it cannot be the sole answer, but I still wonder if it is the largest part of the answer.