Well that's becaue 95% of it is open end and not specific. 5e says "do whatever you want", paartially codified 3 aspects of exploration, and forces DMs to do the rest unguided.
D&D only gets away with it in dungeons because players only expect traps.
Its not just in dungeons, though. The guidance for building an explorable world is all printed, it's just not printed in a way someone that wants to open the book and say "let's prepare 'exploration'" can do in the same way someone can for combat.
Exploration isn't traveling in a dungeon, touching traps, or finding gold. Exploration is interacting with the world in a way that isn't combat or socializing.
The DMG gives a good guide to handling exploration in such regards. They tell you how to build the world, which is important if your players are to interact with it, as well as how to facilitate their interactions. The DMG tells you to start with a home base out of a selection, each with codified population size, local governments, and expected facilities. It also tells you how to create a diverse map with interesting locales and sites.
This fits into the Ranger because a well-prepared adventure will have many places to explore that isn't supposed to be set-in-stone. Even in a story-driven campaign, the adventurers can explore the mountains rather than the forest when going to their destination, while they have a rich history in each.
The mountains may be home to dwarves which ride giant lizards and create magic weapons while the forest may be filled with elves who have aided several celestials in exchange for fruitful harvests. Neither route is necessarily better than the other without the Ranger, but the Ranger can use their expertise within whichever is their favored terrain to effectively resolve a good portion of DM-prewritten conflicts. It's the same principle as a paladin preferring to join a dragon's side in combat rather than the Lich's, one side has a very favorable matchup.
But with a hastened campaign, the forest is merely a forest and the mountains are merely the mountains. Nothing more. In such a campaign, the only difference is the time or maybe getting lost. But with a Ranger, it will feel like a nonissue and the campaign really wasn't affected by the Ranger.
I'm not arguing the DMG did a good job relaying important or complete exploration information. Far from it. I actually dislike the way exploration was handled as well. But I moreso feel that Rangers are more of a victim on the DM's side than just the system as a whole. As such, I suggest better, clearer exploration guidance as opposed to changing the system (via "fixes") or by adding exploration subsections (ala Skill Challenges). What I suggest are more streamlined approaches to making a full campaign world from scratch, though I think WoTC may be more inclined to just have people make crappy homebrews so they can sell modules.
