Back in it's heyday that was the plane of choice, and a great one at that. A better way of asking your question is why do they STILL
The answer to that is those are the most immaculately kept aircraft in the world, so if it ain't broke don't spend billions or trillions of dollars replacing them. Fighters have a high turnover rate because:
A) They get the crap flown out of them pulling Gs etc so they stress quicker
B) If you want to be effective in a frontline wartime scenario you need to have the newest, best technology out there
The 707 fleets of the world's militarys are used primarily in tanker roles, but also other such as AWACS and other AEW platforms. They're not on the frontline (per se) and for a refueling tanker there really haven't been any major leaps or advances in that technology. Sure there have been advances, but not something the would warrant dumping an entire fleet for a newer model. Same for the AEW roles, they only really need their interior technology to be top-of-the-line, the airframe is just a way to move it around.
Hope that helps