First of all , this is an observation of facts in the reality we live in. I agree with the OP argument that these things are intrinsically linked when we just observe the facts without trying to impose our beliefs on them. If we accept that, we can try to theorize about why that may be. My theory is that people are nice when they live among others who look and behave like themselves because that what humans did for almost their whole existence as a species. Archeological evidence suggests that if you met a stranger, someone not from your tribe, chances were high they would be a danger, not an opportunity to make a new friend. So humans seem to have evolved to not trust outsiders, not just strangers. In a small society, conformity is highly valued because doing things without understanding its consequences could often end up in catastrophe. People didn’t understand the world, and other tribes, well enough to have any confidence in whether a new behavior would be beneficial or not. So we end up with humans who trust and depend on others from their own tribe for everything, are highly distrustful of outsiders, and not looking like yourself is the greatest sign that someone is an outsider, and who value and demand conformity as a matter of survival. The world of today breaks some of these expectations in some ways, but they are still mostly accurate in my opinion. People who are more open minded tend to be so because they live in fairly cosmopolitan areas, but those also tend to be bigger cities where you don’t really know most people and where trusting others becomes harder, especially when many people have completely different backgrounds than you.

If you want to assume you can trust your network, so you don't need security in the hosts, you need to have a sysadmin who secures the network or have it airgapped directly. You can get rid of all of that, but then you must assume your network is untrusted and need to have security in every service you run.

Consider what happens if you have a community that is very nice, very trusting, and open to everyone including complete strangers who behave weirdly.

At the beginning it can be a nice place. Until the first abuser finds them. And then... depends on the abuser.

This is not a theory; I have seen it happen, often in groups of nerds who are somewhat naive about how real life works. Then a new guy comes, borrows some money from each of them individually, then disappears. Or a new guy comes, invites a girl to his place, and rapes her. Sometimes some people even get suspicious about the new guy, but is a taboo to express such thoughts loudly, to spread suspicion in a group based on mutual trust.

To avoid things like this, you need to split people into "those I trust" and "strangers". Different people and cultures draw the line differently. For some of them, "those I trust" only includes people they have known for a few years. It is frustrating for a newcomer, and you often wish you were able to do something so that they accept you faster. Unfortunately, if that worked, the abuser would probably do the same thing.

Another strategy is to have mutual friends vouch for someone. But this only works when it is a part of the culture, where people know that vouching for someone brings heavy consequences if that person turns out to be an abuser. In cultures that do not have such experience, abusers often do the trick where they befriend someone, use them to get introduced to their friends, and while the first target is like "this is a nice stranger that I have met yesterday", their friends are like "this is a nice person recommended by my trusted friend, which means I can trust them too", and then the abuser exploits them. (And when they complain to the first target "why did you make me trust such a horrible person", the first target is confused "well, I actually barely knew them, this was a horrible misunderstanding". But the abuser probably has a lot of experience creating such misunderstandings on purpose.)

Finally, you can have a close community that is very difficult to enter. You can trust everyone in that community, because if they do something bad, they will be kicked out, or worse. But this requires the community to be very difficult to enter, and to have ways to punish the abusers in a way that actually hurts them. (Merely kicking someone out of the community is not enough, if there are hundreds of communities, and the abuser simply infiltrates the next one.)

Abusers exist. You need to build a firewall somewhere. There are different places where you can build it, but you have to build it somewhere, or you get hurt.