Because he's bitter. After both himself and his stupid agent were deluded enough to believe Arsenal would even consider signing him.
Given that he's the captain of a team that's just lost 6-0, maybe he should be paying attention to problems nearer home.
I do wonder whether he's got his eye on a career in the media after his retirement. As with the example of Robbie Savage, the TV people seem to be going for a 'personality' who can be a bit 'outspoken', within reason. Talking crap or talking in cliches is not a disqualification.
He definitely seems to be angling for something in the media after he's finished playing.
Him and Robbie Savage to have a show together
They could name it Savage Barton: Truly Deeply Mad.
This deserves recognition.