You're completely right, and it's such a great concept for a campaign that seems so obvious in hindsight that I'm flabbergasted this is the first I've heard it. As a campaign, this provides us a plausible reason for a disparate group of people to band together, a reason for them to travel, opportunities for various adventures along the path, and an end goal.
Or there are odd divides. Like aracane magic is intellectual while divine is completely different. I bet to a lay person, they wouldn't be able to tell them apart. Like American society, D&D has a pretty strong divide between the secular and religious.