There are female rebels in the main rebel camps (i.e. Reddock), they are very rare sight though.
I'll tell you a little secret: Some of the Orcs are actually female. They just sound like male Orcs. Use some imagination and you'll be fine
jk.
My guess of the reason why female are so rare in the game is:
1) They spend all their developing fund on their super-duper-mega-awsome-cali-fragi-funky-sexy quad core quad SLI development PC, so they have no money left to purchase some female NPC resource pack or can't pay enough to the poor 3D-modeler that, he died of hunger before he manged to complete all the mega sexy female models.
2) The developers are gay.
yet again, jk.