I suspect that’s why about all of them do it. It would be interesting to me if some financial experts would thoroughly research the subject and publish a report on the economics of running for a major office. Maybe it’s been done already.
Some average Joe or Jane who might be upper middle class, or just middle class and well-connected in their home district, runs for Congress. In a relatively short time, they become multimillionaires. Look at Ocasio-Cortez, the former bartender who now lives in a luxury apartment in DC. Members of Congress do make low 6-figure salaries, but it's nowhere near enough to become millionaires and afford luxury homes. So how do they become wealthy in a couple of years or so after getting elected? The system is set up so they can game it easily and without fear of the law.