The loot factor is definitely a thing, as you in Skyrim can often find awesome items hidden in a cave. Another factor is that there are so many caves, so when you have finished exploring one cave, then another one shows up on the map and again and again. In RD2 there are some caves and hidden items, but most of the time those are tied to some map or quest that you have to do. I also think it has to do with RD2 having more natural encounters while wandering about. A moose might suddenly run past you, you might find a person in the need of help or some bandits waiting for you in an ambush. The game as I see it also offers more tranquility and serenity, which makes you slow down and suck in all the beautiful vistas and even stay in them for a while. The game offers a different kind of rhythm.
I've been thinking about this for a while. I think I underestimated how much the dungeons in Skyrim are part of the core gameplay.
The main gameplay in Skyrim is to find a dungeon, kill everything in the dungeon and get some treasure at the end, then walk to the next dungeon. Of course, it doesn't have to be a literal dungeon, but most gameplay happens in a pretty linear, interior location. The world map is just there to connect all these interior levels together.
Sure, you can talk to people, but they mostly direct you to go into yet another cave. And sure, you spent a lot of time walking around outside, but besides finding new caves to go into, you really only find the occasional treasure and wandering monster.
What I think The Elder Scrolls series does really well is hiding how much the gameplay revolves around going through fairly linear dungeons. Going from walking around outside to going into a cave is pretty seamless, even considering there is a loading screen. This works a lot better for immersion than games that require you to start a (side)mission by talking to a NPC or activating something in the world, which then switches the game from an open world game to a linear experience by setting a bunch of limitations, mainly on where you're allowed to go and what you're supposed to do.
The Assassin's Creed series are a good example, as there is a clear change between running around in the world and starting a (side)mission, with missions usually giving you a game over screen if you walk too far away or adding temporary (invisible) barriers.
The reason why a lot of open world games opt to switch from a free roaming experience to a very limited challenge is because it's easier to set up a specific challenge if you can control the movement and actions of a player. The beauty of Skyrim is that they limit the movements of the player naturally, instead of setting up arbitrary limitations.
Another thing that I realized is that each dungeon in Skyrim also has to be compatible with every play style that's available. Which explains why the enemy AI forgets about being attacked from the shadows so quickly, as making stealth more forgiving allows for adding extra enemies and tighter spaces that work better for, for example, a berserker style of play.