The issue is unlimited spending. Rich people can tilt the political system to benefit themselves by their ability to spend unlimited and then push for things that enrich themselves like lower taxes that doesn’t benefit society at large.

The biggest example of this in the US is the health system that is more expensive and has worse outcomes than other countries. There is a huge and growing gap in the us between ultra wealthy and the rest of the population and it is a virtuous circle for the ultra wealthy with their ability to spend unlimited in politics.