Let's take an extreme case: every person in the world has their wealth increased by 10% over what they have today, except for one person who has his wealth increased by $1 quadrillion.
Everybody is better off, right? Except that wealthy person is going to rule the world, and stack the deck to help himself, and there's nothing that says he's only going to do it in such a way that everybody else keeps increasing their wealth too, or even keeps their wealth at all.
We can see this at work in the US where the political process has become pretty corrupted, to the point that half the politicians act like wealthy people in the US suffer under a tremendously high tax burden when they actually pay less as a percentage than the average person, and the other half pays lip service to getting the wealthy to pay more but only proposes weak measures that never go anywhere, and where systematic lawbreaking in the financial sector that led to a gigantic financial crises went completely unpunished, except for the one guy who made the mistake of ripping off wealthy people.