there's a lot to cover here if you don't have experience speaking japanese in real life. At any rate, basically these companies are still cherrypicking certain guy voices and certain girl voices, in order to maintain a certain image. It's also pertinent to point out, that japanese anime is still a fantasy world and uses fantasy transcripts, for japanese people, just like it is for other cultures.
Now in real life, japanese people don't sound the same at all, but there is an entire speech/phonation style that differs from other cultures.
On the base level focusing on men, japanese men tend to speak at much higher pitches, however their tones still differ a lot. While a lot of men might be vocally flexible enough, these anime companies are still looking for certain tones, even when those tones are scattered around in a bunch of high voices.
Focusing on women, it's actually more difficult to find good female voices for anime. Japanese women speech is probably much more comparable to other cultures, with some slight differences. Usually, mature/full sounding high or low voices, is more uncommon at the same age in other cultures. These anime companies usually look for high pitched voices that also have a mature/full sound. so essentially, it's more difficult to find women voices for anime. I'm not trying to say anything opinionated or negative here, just trying to be objective as possible in comparison to anime standards.
I hope that explains things good enough that you can visualize it. You really need to hear japanese people talking to understand it better.