Technically women are “kind of” fighting. Since every Valkyrie so far is female they have been fighting with their lives on the line also. And a few have actually died.
But your question probably more relates to “why are no challengers female?”
Answer: no idea. Onto my personal speculation, Probably because since humanity is a male dominant society and historically womens rights have been forced into the dirt (sadly) and this is a historical series, most serious heroic accomplishments were done by a largely male majority since women weren’t always given the chance, nor were probably even honoured as legends, save a few select females worldwide. Ex: Boudicca (my favourite), Hippolyta (more likely to side with the gods anyway and a mythological figure), Fu Hao and Joan of Arc.
There’s clearly room for a few in the roster, but compared to the accomplishments of certain males such as Leonidas, Kojiro Sasaki etc there’s not many options. In addition to this the point of Ragnarok candidates are classed as “Humanities greatest warriors” since historically like I’ve said prior, it was male dominated so was the heroic fan base and their exaggerated accomplishments.
I imagine that could be why.
Way back when in lots of cultures women weren’t even allowed to train with a sword let alone be allowed to fight and gain renown.