There are at least two reasons for this.
First, it seems that human beings are simply greedy. There does not seem to be any way that society can prevent people from wanting more money or more power. This can lead to corruption.
Second, government workers will never get truly large salaries. They may be getting comfortable middle-class salaries, but they will never get rich. At the same time, they will be dealing with people who are rich and who want things from them. So, for example, when a millionaire businessman wants permission for something from a clerk making $30,000 a year, the potential for corruption exists.
Government workers will never be paid huge salaries and people will always be greedy. These two factors ensure that corruption will always be with us.