The reasons are very different than the list above.
I think that governments are always corrupt as a system,
and giving them more power makes the problems greater.
The government structures tend to form a dictatorship around certain subjects.

Giving companies, monopolies or central banks much power can also form dictatorships around certain subjects.
And people supporting socialism are usually against a dictatorship by monopolies.
Sadly this often it replaces it with a different one.
Both seem under control with voting, money or science/religion, but they are actually not.
Dictatorships are all about power.

European socialistic states have some reasonable success.
Sweden did good until they accepted so many immigrants.
Central Europe got mainly problems after forming the corrupt European government (and more immigration).
The reason they did well, was because the corruption grows slower in the rich states.

I think the real question is:
How does a system prevent corruption?

Per system:
How much power and money do you want to give the government?
How much power do the companies have?
How free is the exchange and the production of goods?
And how much is the country under foreign influences?