I don't think there are rules that restrict female from joining male teams.
Simple answer: Not enough talent. Most of the unsigned female players aren't nearly good enough to play on an academy team (most are asc/imm pug rosters), just like most T2/T3 unsigned male teams, and orgs won't risk picking them up over F/A T1 players. Those who are signed aren't going to leave for a highly risky chance at a higher level of competition, or can't leave because of their contracts. meL specifically was offered a trial with KCP before they left the scene, and that's arguably the best player in all of GC.
TL;DR: doesn't make sense for players or orgs to take a risk when they can get pre-proven T1 talent.