You always have a next step, a next upgrade to look forward to. It's an uninterrupted treadmill from building your first mine to building you hundredth mega-factory. That's how even quasi non-games like Cookie Clicker can become addictive. It's effectively a Skinner's box.
Gameplay-wise Factorio is a lot more interesting that Cookie Clicker of course.
Gameplay-wise Factorio is a lot more interesting that Cookie Clicker of course.