A large number (most?) of the H-1B visas go to students on F visas who have a job offer and want to change their status. Almost all the foreigners that I know on H-1B visas in my company came from a F visa. Some definitely were recruited from abroad, but it's simple economics. With so many people graduating from US universities, it's a lot easier to interview and subsequently apply for an H-1B for someone already here.