Title
its because of conflict of interest, there is little to no chance of female & male rosters sharing players but theres many cases of players from main rosters going down to academy rosters and vice versa, it 'ruins' competitive integrity in a way female rosters cant. also Riot wants to give female players more chances to succeed in the scene, and if it wasnt for this rule 99% of female rosters under orgs would not be able to play in the main circuit and wouldnt develop very well
Academy teams are not there maybe, the reasons which comes in my mind are
1)age
2)same org with two teams
3)imagine the academy lineup beating the main lineup (for ex TSM academy beats TSM main roster)
4)orgs have these two teams cuz then when they find a main lineup player underperforming in a role, then person with same role who is on peak, therefore the academy player gets promotion
These are reasons imo