It's not that they have more. It's that you know they have more, that it's not fair they have more because they are not better than you, and the path to get more is blocked / unfairly policed. And that eats you up inside. And that eating up is constantly firing stress hormones and heardening your heart.
In short, anger leads to hate, hate leads to the dark side.
The problem is that, no matter what the actual causes are, self interest bias generally makes it so that nobody can accept that another person worked harder or used better strategy or whatever.
For example, in games, it's always lag/bad teammates/etc. that keep them in "Elo hell" and they want to 1v1 you despite it being a team game and they ignore game winning objectives to score meaningless kills or whatever. Never mind everyone starts with exactly the same resources every time. Yes, yes, bad lag can have an effect on the margins, but some of the best players I knew were playing from another continent with terrible ping and bad lag doesn't have any effect on bad tactics.
I've been in, and climbed out of that Elo in plenty of games, and I know just how sharply the mindsets change as you climb.
> Never mind everyone starts with exactly the same resources every time.
In a game sure, but does this seem to reflect lived reality?
Edit: in fact, I don't know what game you're refering to, but imagine a game where your opponent was given an order of magnitude more HP or an hour to level up/prepare before you entered the game. Is this a game you would want to play?
I don't want it to be about one specific game because it happens in lots of games. There's always someone who will make horribly bad tactical decisions to ignore game winning objectives and blame that on "lag" or something ridiculous.
And the ones I have in mind are such that people do start with the same resources every time.
Though if you want to discuss P2W games, those where actual skill is involved not infrequently have no-skill whales lose to skilled players. In those it depends on the scale of the P2W and how much skill plays into the actual contest, though.
Finally, to your edit, yes, many people play MMOs and even join ones where other players have already maxed everything before they created their first character. Not everyone feels the need to be #1 or even cares if someone has more than they do, because obsessing about that stuff is a sure way to hurt your own mental health.
Then the research is inaccurately described. They should talk about perceived injustice, not merely inequality.
Even the Chinese Communist Party came to realize that people must be allowed to reap unequal rewards (based on their talent and labor) to escape desperate poverty. Their agricultural productivity tripled when they let people sell some of their produce rather than turning it all over to the state [1].
This is how Deng Xiaoping began his rise to power in a country gripped by the madness of Maoism. By accepting inequality even though the Communists have always demonized its socioeconomic manifestations.
In short, anger leads to hate, hate leads to the dark side.