Yes, the NFL is the big league, but it is not the ONLY pro-league, it is not a monopoly.
Most of the small basketball leagues are actually tied to the NBA, they’re the D-league. The small football leagues have no connection to the NFL. And the Arena league and UFL are both national.
The other leagues do not offer a product anywhere close to the NFL even if the odd NFL player decides to play there. NFL product is unique.