Democracy is incompatible with power concentration. Excessive wealth easily translates into power, thus, it breaks the balance of any democracy.
There's also a saying that "Democracy cannot exist while people are hungry", because a common complaint is that "poor people vote with their stomachs".
Let me use an example. Let's say you're my partner, and it's movie night. I give you a choice between two movies: Star Wars, and Harry Potter. However, if you choose Harry Potter for movie night, I will actually break both your hands with a sledgehammer.
I say I'm giving you a choice, but do you actually have a fair choice?
People choose policies that might be beneficial in the short term, but very harmful on the long way. For example, restricting immigration might have a favourable short term impact on wages, but in the long term it will stagnate the economy and pensions schemes, and make people even more poor.
So... People who are hungry will vote for whatever brings food to their table today, so they don't really have a choice, because those policies or politicians are not actually on their side, they are just benefiting from the misery of others.