I don't have one in my own database either.
I think most of the time you don't get any "reasons" for something like that with given names. It so happens over the course of history that somebody coines a female form for
Luke, the name becomes popular and stays - or it doesn't happen, just as well. Nobody will be able to tell why or why not.
A candidate for a female form would be Lucasa, from the Latin form
Lucas, or Loukasa, from the Old Greek Form
Loukas. It's interesting that Search on Facebook comes up with a handful of women that gave Lucasa as their name, so maybe there are female variants after all...
Rene www.BabyNames.ch