Before we even ask what an exploration challenge is supposed to be we need to step back and ask what is the threshold for a challange. Combat is quite involved. But 'the social pillar' is only a pillar because someone says it is. It boils down to rolling skills and roleplaying.
So if I want to sneak past a guard then it's a challenge on the same level of an entire supposed pillar of the game. (I decide what I'm doing and roll the dice and the GM adjudicates based on the result). So why isn't there a stealth pillar?
WotC
You gain XP for recovering lost magic items, claiming hidden treasure caches, and exploring abandoned sites or places of power. Your character can gain experience points by retrieving a mighty weapon from a dragon’s hoard, stealing a diamond from an evil baron, or uncovering the location of a lost temple of evil.
Notice this is what WotC thinks you would get Exploration pillar XP before. It's basically largely indistinguishable from story or goal based XP. And involves activity that almost certainly includes combat and AND/OR social interaction. This is why a dungeon would be an exploration challenge (if anything is - which is questionable).
This means that exploration is not like the other pillars, but then none of them are anything like the others. Actually the worst fit is social interaction, because without it you would have a clear split between combat based/skills and utility based methods of resolution.
We need to be clearer about what we are talking about here. There's no point complaining about the implementation of the exploration pillar and then acting
as if the concept was something much more coherent and categorical then it actually is.
We would be better complaining about the poor implementation of things that make sense. Wilderness survival is largely written out of the game. This seems to be an issue regardless of whatever we imagine the exploration pillar to be. There is no systematic way of structuring non-combat challenges. This is something can be discussed without worrying about whether we think the vague thought bubble about the 'exploration pillar' implies there should be one.
It's not surprising some people think that the 5e handles exploration really well and others think it's awful. What the exploration pillar is really good at is getting people to think they know what it means (which is invariably different to the next person) - it's a kind of vaguely half completed construction to which people can't help attach meeting.