It’s not hard to become a millionaire. Especially when you are famous, and have experience in one of the most prestigious jobs in the world.
Successful politicians are usually smart, and make good advisors for businesses. I don’t believe this is proof for corruption and self-interest.
Provided a successful politician is still working, I actually think it would be hard not to become a millionaire a few years out of office.
They do have the common peoples interest in mind, just in combination with all the special interests they have to please. This, paired with a broken system and systemic incompetence results in peculiar and slow decisions.