I think it's a structural effect. Let's play moral relativism for a second and assume that everyone has their own definition of what is ethical and what isn't, and that people generally choose not to do things they would consider unethical even if doing them would benefit themselves in some way. So, the people with the widest array of options for benefiting themselves are those with the least restrictive ethical framework. This doesn't always mean that they will be successful or powerful, as humans are generally pretty bad at predicting what is good for them, and even worse at consistently acting on those beliefs, especially over the long term. However, the hoarding of wealth has a few characteristics that make it different from other forms of self-benefit:

1. It's easy to measure progress, and therefore easy to optimize for. This means that once you find a successful means of making money, you can fine-tune the process and reproduce it more easily than, say, a critically acclaimed novelist can write a critically acclaimed sequel. (n.b. I'm not saying that getting rich is easy. In fact I think a lot of rich people, especially those at the very top, do genuinely put a lot of hard work and long hours into being rich. I think they're genuinely passionate about being rich. I think it's a selfish and self-defeating and catastrophically harmful goal to pursue, but I think they enjoy it and pursue it with the same vigor that any world-class athlete has for their sport.)

2. Money makes money. This one I think has been discussed enough, but it's an established fact that they easiest way to make money is by having money, which means that people with the most money tend (assuming they don't wildly fuck up, which does sometimes happen) to become even more insanely wealthy. You can even pay people to help your money make money more efficiently, which strikes me as very funny though I can't really articulate why.

3. Having money influences the behavior of everyone around you, whether you want it to or not. The very rich, especially (but by no means exclusively) the famously rich, have their relationships with other people skewed in a very systematic way. This is conjecture on my part, having never been famously rich, but I would imagine that this systematic alteration of relationships is very hard to account for, especially if you get famous before you have a chance to form deep adult relationships. And by account for, I think there are just things rich people do that they simply do not, or cannot, see. Relatedly, I think this is why dictators tend to overreact to political comedians, because that public discussion of their obvious foibles is really the only time they ever hear about it, and it's intolerable because their tolerance for criticism is so low.

I think these traits mean that once you find a way to make enough money to become wealthy, you tend to stay wealthy as long as you can repeat the trick. And since there are tons of ways to make money in unethical ways, loosening or ignoring one's moral compass can greatly increase the odds of finding a repeatable money-making tactic. And once you have a way to make money, the looser restrictions make it easier to grow your hoard faster. Which is why the richest person on earth is invariably some self-obsessed abusive criminal jackass.