Why is it impossible to contract HIV from a toilet seat?

I never use public toilet seats as I can't stand the thought of other people sitting on them and leaving urine or blood stains, I also hate any of my family using public toilets, but now I think I may be taking it too far. My stepson comes and stays every weekend and he has 3 younger sisters (half sisters at home with his mum), he lives in a pretty shitty area of south london and I am concerned that his little sisters sit on some dirty, possibly blood stained pubic toilet seat then go home and sit on the one at home, then my stepson sits on the toilet seat at his house then comes to my house and sits on my toilet seat. I know it sounds crazy but his mum isn't the cleanest of people and I doubt she cleans her toilet seat much (I'm a clean freak!), just worried that he can cross contaminate HIV or Hepatitis. How is it not possible to contract HIV from a toilet seat?