PrinceofNothing said:
War and conflict are much older than any religion. Again, show me proof of a female led Military if you want me to take you seriously.
Joan of Arc, whom Saskia is an obvious allusion to.
*Puts on Biologist's hat*
Genrally speaking, males are better at physically demanding tasks than women. There are female outliers- by inclination, natural ability, training, and so on but even then they do not reach the same level of ability of a male who is good but not great in the same field; consider Olympic weight lifting, sprinting, marathon running, and so on. The most exceptional males drastically outperform the most exceptional females.
This being said, in tasks that require an application of skill the difference is much less pronounced; I would go so far as to say completely negligable. A skilled archer is a danger no matter the gender (though a male will probably be able to use a bow with a greater draw strength, which will result in greater range and potentially more forceful shots) and if considering the use of martial arts various nerve strikes, joint locks, trips, Ju-jitsu style force redirections and throws and so on can be used to devastating effect by trained combatants.
In the case of a Witcher, they usually go in having done their homework and with some awareness of what they will be facing. A simple killing contract typically involves an ambush (or other surprise attack) of the monster while under the influence of what amounts to combat drugs, with a short and decisive (one way or the other) battle as a result. So endurance may not be that big a deal, either- assuming Witcher-level combat prowess and preperation, ideally the fight will be over before it becomes a factor.
However, you cannot absolutely garuantee the element of surprise, so the greater baseline physical capability of males may be more desirable in the aggregate, especially given the difficulty in creating a Witcher.
As to why there are no (known) female witchers, I would say that this is due to both social mores and simple biology. When the Witchers were created, it was essentially so that the newly-arrived humanity would have expendable shock troopers who could take down post-Conjunction monsters that threatened the race. There were males who were, shall we say, "surplus to requirements" and this gave them a way of being made useful without having an adverse effect on humanity's chances of survival as a whole- quite the reverse, in fact.
The mutagens used in the first phases of the Trials seem to have negative effects when applied to females, which is why Yennefer made the Witchers stop the process on Ciri (not that they had any idea about how to complete it, much less access to anyone who could oversee it); it may be possible to use alternatives that don't have these side effects, but if the outcome is lessened capability or an increased failure rate this might not be a desirable option.
By Geralt's time, the Witchers are a dying breed. The Wolf School consists of just Geralt, Vesemir, Eskell, Lambert, and Coen in the books; the first game added Berengar. Everyone else was killed during the Kaer Morhen uprising several decades ago. The Viper school- at least everyone that Letho could account for- had a similar size, consisting of Letho, Serrit, Auckles, and the un-named Viper Geralt kills in the end cinematic for the first game. Two other Schools- Griffin and Cat- are known of only because Leo Bonhart had medallions from Witchers he'd killed; as no other Witchers from those schools are ever mentioned it's possible he killed the last members of those Schools and that none remain.
Mages who understand the mutagenic process are also in short supply. It seems that creating a Witcher would require Kalkstein-like alchemical knowledge in addition to Yennefer-level mastery over biology. Individuals with such capabilities are not especially common, and they may very well have to start over from scratch, with a lot of trial and error. When the "perfected" process performed by those with the required abilities and experience had a 70+ percent failure rate, this does not bode well for the creation of new Witchers.
This, however, leaves out the question of motivation. Successive generations of Witchers over the past 500ish years- coupled with other changes to the world following the Conjunction- have greatly lessened the need for Witchers. Where once a super-soldier type might have been needed, the monsters that remain are far less dangerous and can be handled by simply piling on enough soldiers; the Order of the White/Flaming Rose being a good case in point here. So the effort of creating a Witcher could probably be put to more effective use by training large groups of unaltered humans, with the added benefit of getting a standing army out of the deal.
There's also the matter of the part destiny plays in the creation of Witchers. At least in the first game it's implied there has to be something special about potential candidates, which is why Witchers often recruited via Surprise Children.
*Takes off Biologist's hat, puts on Literary Critic's hat*
Whatever the in-universe or in-character explanations, the most likely reason that there aren't any female Witchers is that the books are based in a Patriarchal, European-like fantasy mileau. One of the standards of the genre is the male warrior hero; given this it's notable that, while we don't see any female Witchers we do see quite a lot of competant, dangerous females. White Rayla and Ves might "only" be human females, but they are highly lethal and there are similar characters like Calanthe in the novels. Toruviel gets the drop on Geralt and Dandelion, and no matter how dangerous a single Witcher is they cannot devastate entire armies like Sabrina Gleveissig did or hold a candle to even Triss' power level.
There are enough examples of females who can perform at the rough equal of Geralt's level and/or better that I'm inclined to give the issue a pass. Your mileage may vary.