What you’re describing is essentially how society functioned for most of human history until the advent of large-scale, organized civilization. In fact, even in modern society, most family structures and friendship dynamics worldwide still use this.

Mutual aid depends upon reciprocity, the idea that I help you today on the understanding that you’ll help me when I need help. But what happens when Gondor calls for aid but Rohan refuses to answer, especially despite being capable of it? Well, at a small scale (such as within families or among friends) we make a mental note to never help that particular person again, and we might even tell others that they’re a selfish scumbag and not to trust them. Therein lies the disincentive to cheat: if you cheat, you will find yourself cut off from any future aid.

But humans have a fundamental limit to how many personal relationships we can maintain. In fact, this concept has a name: Dunbar’s Number.

Dunbar’s number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships—relationships in which an individual knows who each person is and how each person relates to every other person.[1][2]

It has been proposed to lie between 100 and 250, with a commonly used value of 150.[8][9]

So if your society/family/mutual aid group gets bigger than Dunbar’s number, you stop being able to meaningfully keep track of who’s a cheat, and so does everyone else. Without that, cheaters can cheat without nearly as much repercussion, which breaks down the whole system of trust mutual aid is built upon.

And that’s why we now have more complex systems in modern society. Things like currency and laws to ensure fair exchange and stop cheaters. But even with all of this, we still have mutual aid in the form of our immediate friends and family. And because friends and family are a naturally select group, they’ll never surpass Dunbar’s number, allowing the system of trust to (mostly) function. It still sometimes breaks, of course, such as when families excuse toxic behavior so as to not rock the boat.