Because Wenger sell them when they are past their prime. I mean, I wouldn't have known Henry had lost it, I thought he only had one off season with a lot of injuries and that he would eventually be back to his former self. But apparently Wenger knew Henry would never be the same player again. Same with Vieira, Pires, Ljungberg. He cashed in ridiculous amounts on those even though none of them would ever again be as good as they were at Arsenal. In short, Wenger is a genius, but that's no news I suppose.