He probably continues to tour because he still has fans that want to see him.
I'm not much for concerts. . . too much money, too many people, too loud, etc. There are two living musicians that I really want to see perform-- Paul McCartney and Sting. I got to see Sting last year, and if I can see Paul McCartney this year I will be a very happy person.
Every time he does a tour, I think, "This'll be the last time." If he is doing a 2009 tour, I'd be glad to go! He has just released a new album so it would make sense if he is out touring again.
Well, count me in--if I can ever find where/when he will be close by!