Because Skyrim feels infinite. Like, each new playthrough is new. I'm a new character, and I have new things to do, and everything changes based on what I do, how I do it, and when I do it. Plus the world is just- like you point out, the world is incredible. It's been seven years (!) and to this day I haven't found a single game world that will engage me like Skyrim does, not even Hyrule in BotW.+1
How do you able to replay Skyrim that much? This is an honest question since I'm always baffled with people seemingly able to replay it so many times while I'm not. My first and only playthrough lasted for 100 hours, loving every second of it. Since I played it during launch day, I only installed few mods along the way. When I consider myself to be finished with my first playthrough, I installed many mods from texture, weapon packs, to overhaul. But I never get past the tutorial stage. This happened again, and again for like 4 goddamn times. My burning desire to try modded Skyrim is extinguished as soon as I remember the quest quality and the fact that I finished most of them, sparing only the Thieves Guild line on purpose but that single quest alone didn't managed to hook me even past the tutorial. It turned from my GOTY of that year, into my most mortally hated game of all time. I sworn I never play Bethesda game on release again so I can play it to the fullest with all shit being fixed by mods.
How do people keep playing it, especially on console when there is no mod to spice things up and fix shits that should be Bethesda job in the first place? I admit, the world design and atmosphere is astounding which makes exploration feels as a reward on its own. Even when the landscape isn't as diverse as Cyrodill or Morrowind, the northern vastness of Skyrim is still as much as engaging. But the thing with exploration is that they only lasted for your first or second playthrough, and any subsequent play after that need something else to hook you up.
I don't know. Maybe what I am saying is alien to you, because you feel you exhausted everything the game had to offer in those 100 hours. Me, I still don't feel I am anywhere close to done. And that's why I keep going back to it.
I don't know how much I answered your question lol