I disagree entirely with your statement that: "This isn't human nature, but is simply accepted and defended behavior that gets caught out again, and again, and again."

This is patently false - remembering a different password for every single system, device and site you interact with is not a feasible proposition for the vast majority, especially if you require these passwords to be in any way meaningfully secure.

There are ways of sidestepping this problem, such as 1password and the like, but the ones that are most seamless are paid for services and hence the adoption rate among technically illiterate people is pretty small (I'd imagine, no stats here).

The real issue is that passwords are a broken way of authenticating. End of. Passwords that are easy to remember are trivial to crack, and passwords that are difficult to crack are hard to remember. This is the issue here.

People may do dumb things, but it is far easier to change your system than it is them.

It's one thing to argue for improving people's password practices, but please don't pretend that there's no reason for their behavior. The vast majority of people who share passwords between sites experience no repercussions from their choice. And choosing not to create a new password for every site saves them time and potential frustration.

That's the human nature part, to assess the risk of behavior and change it only if future experiences show that the costs associated with that behavior are too high. Since most people don't experience the disadvantages and do experience the benefits this behavior continues.

We can encourage more people to avoid this behavior by explaining the potential impacts and providing accurate estimates of the risk they're taking. We can offer alternatives to password reuse, like using a password manager. But ultimately they are still going to weigh their perception of the risk and benefits to make their own decision.