I can see every single example taking that much time, but Skyrim? Is there really 3000h+ worth of content in that game that doesn't get boring after first time you do it?
I feel like you need to be introduced to mods. There are...a lot. Even if you keep it to just relatively high quality ones that add content (rather than mechanical overhauls or graphical overhauls), there are still a lot.
I'd suggest Falskaar, Wyrmstooth, The Hanging Gardens, The Maelstrom and vicn's mods (Vigilant, Glenmoril and Unslaad) as a starting point.
Actually, that's not true, I'd recommend Legacy of the Dragonborn as a starting point, then grab mods that require it and mods that require those until you have all the content mods that can have displays in the museum (which includes all the ones I mentioned before, but is not limited to them).
The other gut hit the nail on the head. It's the modding. Skyrim is a sandbox game, but when YOU get to make/ customize the sandbox, that basically becomes a game, in and of itself.