They don't want kids playing pornographic games. Probably the same reason every other company does it: Apple, Google, Sony, Microsoft, etc.
I was 11 years old when I first looked at hentai online. It didn't disturb me at all unless it was the violent rapey stuff. Which admittedly is far too common in hentai. Not because it is sexual, but because it is violent.
I would have had no interest in ultraviolent games at all.
I'm not saying it is right or was okay for me to do. But I did have biological urges and sought out eye candy. It was a biological urge that I sought out. Even at that age.
Whereas I
always thought violence was disturbing.
If you were to ask 11 year old me what was more disturbing, ultraviolence, or the sexuality present in some Steam games, I would say the non-violent sexuality, no question.
And if kids, like I once was, would have access to sexual content on Steam, without these measures, they still have access to horribly violent content.
Childhood me would have been horribly disturbed by some of the violence on Steam.
I'm not trying to justify what I did as a hormonal kid, or say that this is okay. But simply illustrate that, if kids shouldn't have access to sexual games on Steam, they shouldn't have access to disturbing violent ones, either.