Why?
Easy.
Springsteen & The E Street... the Who... the Stones... all are still operating well into their 60s and 70s, none have the original lineups still in place. Key figures yes... everyone no. But they can still go on as long as Mick, Keith, Roger, Pete and Bruce are involved.
Can you see U2 going on without any one of the 4? Of course not. And I'm not necessarily talking of death... but the lead singer has had serious back surgery and sinus surgery, both of which could have stopped the band dead in its tracks right then and there. Larry has had various physical ailments that come with hitting things for a living, and The Edge has had an incredibly sad experience with his daughter that delayed a tour twice.
They have the highest grossing tour of am time, a record that can probably only be topped at this point by themselves. They have immense wealth and fame beyond their wildest dreams, and they have families that are embarking on their own lives and careers now.
Add that with the idea that the NEXT tour will have them in their 60s, and an ever changing musical landscape that frankly has them on the outside looking in?
Why wouldn't they think of hanging it up?
Now I for one don't think they will retire after this cycle. I do, however, think that this is their final attempt at so called world domination, and that after this, things will be different. But even so... on the last tour, I approached the final Meadowlands show with the idea that it may be the last time I get to see them. I'm glad it's not, but every tour from here on out presents that scenario as a very real possibility. To think other wise would be naive.