Your question seems to indicate that you aren't from the United States. College football is just one sport among many popular college sports in the U.S. It be

Your question seems to indicate that you aren't from the United States. College football is just one sport among many popular college sports in the U.S. It began as a club-type extra curricular activity but as it became more popular universities likely saw it as a way to market their school and attract customers (students). The top 30-40 programs produce $100 million in revenue which is usually enough to be self-sustaining and help pay for all the other sports teams on campus. College basketball, baseball, football, track and field, and even hockey in some places are de facto minor leagues. The power and influence of the NFL is probably more to blame for the lack of a proper minor league in football.