Because they make *MONEY* from controlling the government. All that graft, corruption, bribery, selling of influence, administering government programs and so forth, all of that enriches the cartel that administers these programs and the companies that buy congressmen and also the congressmen themselves.
How did AOC go from completely poor to now worth 10s of millions of dollars? How did Nancy Pelosi get 120 million as a congresswoman?
They do favors for corporations, foreign governments, foreign corporations, and anyone else who bribes them.
The system is rotten, and it has been rotten going back at least to the 1850s, and probably all the way back to the 1820s.
Yes, there's some truth in your arguments, but also fundamental flaws and those begin with your efforts to impose your own very high moral standards on people who do not, or did not, recognize your standards as legitimate.
It's a flaw you well understand regarding, for example, our slaveholding Founding Fathers, but when you're own favorite ax needs sharpening, you're OK with using historical congress-critters as whetstones.
So, your basic root problem here is that Congress itself, not DiogenesLamp, defines by law what is, or is not, "corrupt" and so far as I know, Nancy Pelosi never once wrote a law which made her own wealth-grabbing activities immoral or illegal.
And if you don't believe me on this, then go and ask her yourself.
As for when did your definition of "corruption" begin in Congress, I'd naturally assume it began in some form on or around Day One, in 1789.
Why would you expect anything different?