Stark got SHIELD because Congress is run by idiots and the superhero community had a collective idiot ball moment.
Also, Stark is NOT a good business man. He's a genius inventor, but a terrible executive. He's run his company into the ground several times and had his designs stolen by numerous indivuals. His reputation is because he keeps salvaging his companies from the brink of disaster or started up new ones that outdo the old ones. Tony is an inventor and THE MAN when it comes to human-tech powered armor. but as a business man? He couldn't have pulled off Bruce's coup of Lexcorp. Hell, Lexcorp probably would have bought out Stark Enterprises three times over.