The main quest, like I mentioned in a previous post, is only a handful of hours long. If that's all you do, then I could understand not getting how people can sink that sort of time into a game.
However, outside of that, there are something like 200+ sidequests, if you're on PC there are mods that add entire new landmasses with their own quests, so you could easily sock away 100+ hours on just ONE playthrough. But many people like trying the game in different ways - stealth archer (as if there was any other way to play!), sword and board warrior, mage, healer, there are so many different ways to spec your character, and each of them makes for a different game.
I did actually go through a game once where I had three companions, and I was a healer - it's a lot more challenging than you may think, since you can't see health pools like you can in, say, an MMO. But it was a lot of fun. I never actually killed anything unless I absolutely had to, I just let my companions do it.
Of course, mods add so much more life to the game than what you get out of the box. New quests, new dungeons, new houses, companions, bosses, monsters, loot, you never play the same game twice.
If you're one of those people who staunchly refuse to play anything outside of the main story, The Elder Scrolls games are definitely not for you. But if you love just going off the beaten path and finding shit to do, you have an almost never ending supply in Skyrim.
My biggest beef with the game is that they apparently hired many, many more voice actors than in any previous title, but half the fucking people you run across all sound like Ralof >.> Hell, following the main quest at the start, if you choose to follow him, he ends up talking with an NPC that has ithe same exact voice actor, so he's basically talking to himself!