You can't go beyond 10 so far, the same as you couldn't go beyond 10 in relationships until it got expanded to 12. And there has been several "soft" limits since the beginning where you didn't get an attribute increase if you already were beyond a certain number that got expanded chapter by chapter. I see no reason to necessarily assume 10 will be the hard limit for the whole duration of the game.Because they are? It's in the code, I think. You can max charisma in the last chapter and after that if story progress awards additional points to that branch it doesn't add anything beyond 10.