Exact reason football and NBA don’t do it - franchise owners don’t want to lose money on it- they rely on colleges for it. MLB farm system has gotten smaller. In 2021 they cut 42 franchises. NBA’s average g league attendance is around 2,000/ game for their 31 teams. They even have a few clubs averaging less than 1,000 per game, the new G league ignite only averaged 2,700 last year. Average attendance for minor league baseball is over 4,000 per game for their 120 teams. College football and basketball teams were prevalent in colleges well before viable pro basketball or football leagues unlike baseball which had both college teams and pro leagues from way back in the 1860’s.