I quit Skyrim about 8 months ago, partly due to time constraints, but mostly boredom.
Last month I downloaded the Creation Kit, and learned how to mod, and write some scripts in Papyrus, and look up mods on Nexus.
Anything I was annoyed by in the vanilla game can be fixed, tweaked, or revamped via existing or my own mods. Additionally, there are dozens of mods out there that change the gaming experience for the better. The best part is that "better" is entirely subjective, and there are still mods that make it "better" by any definition.
In short, even if you don't care about it at first, modding can add literally dozens, if not hundreds, of hours to the shelf life of the game. I would probably not be playing anymore if I couldn't make weapons where power attacks shoot light beams that stagger and burn random enemies. If you hate the magic system, like I do, there are simple mods that fix it. If you're annoyed that most of the gear you find is useless, there are mods that make you want to collect random gear to melt down into crafting components. If you're pissed that you need to constantly wait for 2 days between selling your junk to merchants, there's a mod for that, too. I honestly can't imagine that anyone would be interested in the game for more than 3 months without modding.