That's just not feasible or desirable for some groups, particularly if they are trying to experience the storyline of a published module or a plot written by the DM who doesn't want to throw his or her prep out the window.
Fortunately, the players have more or less chosen which path to follow, of several I have presented, and all I'm doing is building the road in front of them. On the occasions when they've been between paths, or in a position to choose a new path, I've had to do more ad libbing, and on more than one occasion I've told them straight out they'd outstripped my prep.