To give an explanation that would satisfy me, I'd have to drag in graduate-level comparative physiology, and I wouldn't like to sound any more pedantic than I already have
If you spend a lot of time looking at humans or animals as biological systems and how those systems work, you see some often-repeated themes. One of these, and I'll concentrate on it, is amplification. It's how a small event produces a big response. For example, if you get a paper cut on your finger, there's a cascade of at least ten stages (depending on how you count them) from noticing that a blood vessel has been damaged to producing the proteins that form a clot and stop the bleeding. Each of these stages allows a small but detectable stimulus to produce a larger, carefully regulated, response. If any of the stages is falsely triggered, blocked, disrupted, or wrongly regulated, you may get a disastrous failure to respond (hemophilia) or an out-of-control response (say, "economy class syndrome").
In this way, differences that do not appear to amount to much between species, between individuals, or between sexes may cause wildly different results when the systems they're part of are pushed in ways they haven't evolved to be pushed. This doesn't say anything about one being better than the other. Just that they're different, and the differences can't even be guessed unless you already have models and theories that are well tested and known to predict these sorts of differences. Some of the differences between men and women that make a difference in an athletic field such as witcher training are the patterns in which they store and metabolize "essential" fat and the ways in which they regulate body temperature when exercising or when exposed to cold.
Back to the Witcher world, we have (or had) mages with extensive experience, hundreds or thousands of subjects over many tens of years, in the physiology of human boys and what the effects of well-regulated dosing with mutating drugs would be. Even so, they know (knew) them to be often fatal, or crippling, or leading to insanity.
They could assume girls would respond the same way, and if they got an orphan girl to train up as a witcher, they might treat them in the same manner. It's my contention that this would not be prudent, it would be likely to fail for reasons they could not explain, and they know (knew) enough to think it dangerous and not make the attempt.