The crawl IS the actual story! It's the story of what you're doing in the moment right now to a) survive and b) achieve whatever it is you're there to do; along with what you're saying as you do this.
Mapping, exploring, searching, wandering monsters, traps, set-piece encounters, field-testing magic items, sorting out treasure - all of these take time, and that time adds up. And that's before time taken for inter-character role-play and discussion, PC romances and rivalries, arguments or conflicts, etc.
Never mind that oftentimes an adventure's relation to ongoing stories (and my games usually have several not-necessarily connected stories going on at once) might not become apparent until months or even years later. Sometimes it never becomes obvious, particularly if whatever story the adventure's supposed to be tied to for some reason later never gets off the ground or becomes relevant - not every story arc captures people's interest.