The “bad” ones just float to the top when a government is around long enough. It’s human nature. The ones seeking power and money see others seeking the same, and they look at them as a rung on a ladder. They help each other because it benefits them. They actively work to shut out the “good” ones because they know they’ll lose their spot to them if they aren’t careful. Suddenly the “good” ones are lost in the noise. The incorruptible become powerless.
It’s similar to the police force in the US, for example. People go in with good intentions, but the force has been around long enough that the mad dogs have already permeated all of the top ranks. So the “good” ones either wash out, or they assimilate. One of my relatives washed out, while the one that had been there much longer turned into a massive racist. Which one do you think climbed the ranks quicker?
Power structures automatically attract selfish, self-serving people that just become worse offenders the more wealthy and powerful they become. Power is a vacuum after all, and there are plenty of bad people standing in line to fill it. This is why I don’t trust a single politician. Whether they want to admit it or not, under that facade of wanting to change their city or “do good,” there is some underlying desire for power and attention. That doesn’t mean none of them do anything good in their time though; it just means we should be cautious.