Even if most people are selfish, Mutualism tends to reward the least productive.
A worker-owned factory has 1000 workers producing 100 X each, a total of 100,000 X. Each worker is given a set share of the "profit" of this factory, if you could call it that, and votes on issues of capital allocation.
Yet (A) There is no incentive to invest in capital expansion, risky technologies, etc because each worker naturally wants a more secure position rather than risk (when you have a Capitalist Exploiting Pig, he takes that risk so the workers are still getting paid the same amount even if he cocks it up and loses a ton of money), so any "successes" go into higher wages/benefits rather than capital, so production across the world is lower (Note how most "Worker owned factories" that succeed are in places with terrible economies and little capital accumulation eg. 1930s USA or modern Argentina) and (B) Each worker, even if he tends towards "goodness", is rewarded for being a slacker.
So I'm working in this factory, and let's say I work my ass off until I practically die of exhaustion to increase my productivity by 100%, meaning I am now making 200 X. The factory overall, however, is now making 100,200 X, so I am now making .2 X more in pay for 100% more productivity. Meanwhile, if I slack off and produce nothing, or am even a shitty worker and break things, the factory now has 99,800 X, and my "pay" is now 99.8 X. So in other words, I have absolutely no reason to put in any effort beyond the appearance (so as to not get voted out of the factory) of effort unless I consider the value of .2/.4 X to be worth putting in significantly more of an effort.
Now there are other ways to "run" things besides mass vote in this way and equal pay for all workers, but when the workers themselves are voting on what they want, they'll naturally tend towards forming cliques to get themselves big paychecks at the expense of others, even if they're good-intentioned.