Well take that CR 15 dragon, for instance. He's not alone, just sitting in his cave. He has minions out raiding the land around. And the low and distant minions are the enemies the PCs take on at low levels. Then as the PCs gain power and knowledge of the dragon's "operation", they can take on more powerful minions---leutenants. Then they will move up the "food chain" and eventually right up against the dragon itself. And after defeating the CR 15 dragon, they learn that the dragon was actually working the kingdom on behalf of a demon prince. Etc.
Now, if the PCs decide to go right for the dragon first, at low levels, they will either bounce off the tougher defenses, or they will die.
Novice mountain climbers don't attempt Everest. They work the smaller mounts, then the larger, then the greats. Same with adventurers taking on adventure.
Quasqueton