Well you don't know that for sure actually XD

And I got many points to prove otherwise because I did research into the game

As you know, the ghosts are the ones who control the animatronics and make them go after the security guard for revenge. And Freddy just so happens to enter the girl's washroom in the first game. I wonder who could've thought to do that. (Hint, the one who made the animatronic kill

)

If you speed up Freddy's laugh, it sounds like a little girl laughing. Now, you shouldn't expect a three year old to speak their voice when possessing an animatronic with its own, different voice. It would sound odd, and Fredy's laugh IS kind of odd. And Scott chose the little girl laugh and slowed it down on purpose even naming it little girl laugh in the files. But we'll never know why, I guess.

Third, in the book silver eyes when Charlie does a reunion with her old friends for a memorial of Michael (Golden Freddy's ghost), Michael's parents speak of the incident saying he went missing along with two other male children and two other FEMALE children which means Chica can't be the only female ghost. Or maybe Chica never had a female one and its Foxy or Bonnie.

But Freddy definitely has most proof for a certain ghost gender (female XD) than any other not counting Michael lol, so there you go. This is why I, because I wont force you to believe something if you have a headcanon, believe that Freddy's ghost is indeed female.