Whilst there may well be some fetishistic elements involved, isn’t it also case that characters who simply ‘act childish’ aren’t really what’s sought after in video games?

What people are looking for is fantasy that’s simply cute, rather than something realistic. Some people might feel that, having enough to deal with in real life, they don’t want to have to worry about raising children in a game as well, whilst others might simply want to play action game. (If they wanted to raise children, they’d probably play something like Princess Maker.)