I don't play Rust or DayZ (not a PC gamer), so I'm not understanding something here. Wouldn't cooperating with other players be beneficial? More zombies than players right? So wouldn't it be like the Walking Dead when Rick figures out that more people is better because when you have plenty of people you have more skills/hands/eyes/brains available to do stuff?
So why aren't smart players camping spawns to recruit new players into groups so they can secure larger areas and more resources, and then when group borders meet up, merging groups into town-states.
The short answer is that some people are just assholes.
The long answer is more complicated. In a game like DayZ, you have to work to get equipped, to get food, to find shelter. At any time you can be killed and have to start all over with nothing. Now, imagine you come across a village that you want to scavenge. You can go in, risk getting attacked by another player, risk getting killed by zombies, and you don't even know if there is anything worth the risk. But if you see someone else searching the village, and say you have a rifle, you can just sit back, let them take all the risk by clearing the houses and grabbing anything good, then once they leave you can simply shoot them and take whatever gear they found along with whatever they had on them. You have significantly reduced the risk to yourself by doing so and get to live a little bit longer. Plus, since when you spawn you spawn with essentially nothing (just a flashlight), camping a spawn location (which is really the entire coastline) doesn't really help yourself since you would have to give things to your new buddy for him to be of any immediate use, and then would have to split whatever you found later with them as well.
It's really just history repeating itself. People do work together but only in the small groups of people that they trust. Tribe mentality takes over and outsiders are killed on sight because they can't be trusted.