Or, compare these two scenarios:
I need 5 more gold to buy an item. I ask my partners for the money, and wait a while, and eventually they tell me they "need the money" because they are saving for something they'll get later. I explain I'll pay them back soon. After a long delay, and wasted time, I give up. I go kill stuff and get 5gp, walk back to town, and finally get my item.
Alternatively, a partner gives me 5gp right away. I get the item, use it to kill stuff faster, and then pay my partner back, and need not return to town.
(Not that paying each other back should be important, everyone should just give all their money to whoever happens to be at a store ... but that's just too much to expect of random people.)
Another way morality helps, is over the course of many games, moral people improve more. They are accustomed to solving problems, and when something goes wrong, they figure out how to do better next time. Alternatively, some people, upon failure, get mad and resentful.
Like, some people think advice is an insult, as if they aren't good enough. Well, truth is, they are not perfect, and their arrogance only makes them stay bad.
The effect of all this is so great, that simply by figuring out what to do, sharing gold, and coordinating our efforts, my friends and I can easily win with 2 or 3 players vs 5 players on certain maps.