Let's take a closer look.
Deus Ex isn't even a sandbox game, so that is out of the picture.
Yes, there are lots of things to do. Unfortunately in Skyrim, 90% of the time it boils down to "go to dungeon x filled with draugr and fetch me some item you don't care about.
I don't think you truly know what you are implying. Game AI isn't anywhere close, especially in Skyrim and Fallout.
Yes, look at these
near sentient NPCs.
What decisions? Whether to kill the draugr in the cave and grab the item, or whether to kill the draugr in the gave and grab the item?
Again, what reactions? This isn't Bioware we are talking about.
What narrative? There is barely any.
"I used to be an adventurer like you. Then I took an arrow in the knee..."
"Let me guess... someone stole your sweetroll."
"You see those warriors from Hammerfell? They've got curved swords. Curved. Swords."
Great writing. 10/10. So witty.
All in all, they are good games. They fill their niche. But they are nothing as you describe. The AI is extremely shallow, the story generic, the world static and nearly indifferent to your presence and progress. What those games offer is a lot of content, even if they content is shallow. It is quantity over quality, and freedom of pacing.
Someone may play Skyrim 60 hours, another 100, and another 300. The difference between those people is how long they can do the same thing over and over before they get bored. You see more of Skyrim's content in the first 10 hours, but how long before you get tired of it? The game will spit out content at you until you give up.