Because off the "explicit" nature of the game having children perceived or even miss-perceived could open the developer open to all sorts of legal and or social real life ramifications.
If one takes an approach to children the way they did in Skyrim it probably wouldn't lead to any issues.
The children in skyrim are made untouchable by the game developer.
But there is a more important factor they fill a role that can't be filled by adults that is of child orphans and children that can be adopted.
There was two primary reasons for the children being made untouchable in Skyrim. First, was the violence but also because the games are known to have mod content that is sexualized.
That said there isn't really anything preventing the children from being modded or even having had them added into the previous games that didn't have children. I know for a fact mods have been made for at least violence and murdering the children in the game. I can imagine there are people who have made other kids.
Bethesda would use the same excuse in both. There is technically nothing that prevents any game from being modded and having content placed in it the original developers didn't intend. You can't blame the developer for another bad actors actions.
That would be the same legal excuse in this.
Technically clones are another story. The laws on the books don't look at metal age they look at physical age. So even if the clone is of a fully grown adult and has the looks and appearance of such the law would only legally consider the physical age.
If the clone was 10 days old since it was made and it looks like it was age 30 wouldn't matter it is 10 days old.
So the way the game is now can legally be just as much of an issue. That said I'm not familiar with any court cases that have approached the issue. I think most prosecutors would consider it a bad case to take on. That said it doesn't mean the law allows for it.
Another reason they probably don't want to try the case has to do with the ramifications of if they lost and what could be opened up by it. Suddenly you would have a case that says the physical age of the person is less important than the mental or physical appearance. That would cause issues with child sex offense cases.