As a person who has spent over 300 hours in factorio, I can tell you it's incredibly engaging. People can say whatever they want about story-focused games being immersive, but they are like reading - you are just the observer of a predetermined story. Even if the game tries to make it feel like you're taking part in the events, it's all just an illusion and feels artificial unless done extemely well. The modern AAA games take it up a notch, but they can't truly get rid of that fundamental problem.
Games like Factorio, meanwhile, are different in the way that they don't tell you what to do. Everything you do is your own decision and your own accomplishment. Building new things, you see your own progress - unlike some statistics or a next cleared mission or area, you actually see and make further use of the result of your work. That makes them addicting and engaging.
Then, if you add the fact that games of that kind are replayable, you get the result you see.