Just wondering, but why is it never 'All-Mother'?
One reason might be that all of the playable Demigods are half-siblings. All of them have different mortal mothers and backgrounds but all have the same divine daddy - the All-Father.
How would it be if it was an All-Mother, who had a whole bunch of kids off of different dads but didn't stick around to raise any of them?
(Actually this would be kinda interesting and subversive. Replace Zeus with Lillith and her brood)
Point is, it's a lot more comfortable, culturally, to have an promiscuous and absent dad than a promiscuous and absent mom, mortal or otherwise...