Aiming for a mutually beneficial interaction is often good regardless, but then the question is: do you feel any compulsion to follow through? Or to be forthright?

For example, suppose you and some other random players decide to farm a boss that occasionally drops armor pieces until each of you have a full suit. "Let's farm Xyblax until we have full iridescent sapphire gear!"

Okay, cool, mutually beneficial.

Either through rotation or random assignment, you get your gear first. Bye! You're out. No reason to farm Xyblax any more for you. And because you're from the Kingdom of the White Blades, and they're from other kingdoms, the chance you'll run into them again is roughly zero.

If you have the altruistic instinct, you're like, "No, we're gonna get everyone their gear! We all agreed!" But the egoist, at that point, is like, "hey I got mine, and they got something too...was win-win before but now it's not so I'm out".

Now, maybe the other players are worried about that and you "promise". But it's just words, not a legally binding contract. If they believe you, it doesn't change the calculation at the end. You still dump them. They can find some other rando to finish farming.

Because (public) corporations are legally obligated to be sociopathic egotists, they write all that stuff down in a legally binding contract that is enforced by a higher power (i.e. government). Of course Exxon-Mobil is going to dump you once it gets its final armor piece! Please. What are you expecting from them, charity? But writing down legally binding contracts for interpersonal interactions is exhausting and also is a very clear sign that something's likely wrong with the altruistic instinct, which makes many simple interactions fraught.