The reason is simple: through the magic of mechanics, each gear runs at 1/50th the speed of the previous gear. A Redditor made the calculations: the engine runs at 200 revolutions per minute moving the first gear at 4 revolutions per minute through the worm gear at the end of its axle (which doesn't count as part of the 12-gear mechanism). The second gear then runs at 4.8 revolutions per hour (which explains why you can barely appreciate any movement). Then things go from glacial to geological to galactic-scale speeds:
Third: 1 rev/10.4 hours
Fourth: 1 rev/3.1 weeks
Fifth: 1 rev/2.98 years
Sixth: 1 rev/149 years
Seventh: 1 rev/7452 years
Eight: 1 rev/372,600 years
Ninth: 1 rev/18.6 million years
Tenth: 1 rev/932 million years
Eleventh: 1 rev/47 billion years
Twelve: 1 rev/2.3 trillion years
That's the reason why the entire machine looks still and the last gear is locked to concrete without any effect on the entire mechanism. In reality, every gear is moving even while we can't see the motion. They just move too slowly—so slow that the last gear isn't affected by the fact that it is attached to concrete. To a human, everything is effectively frozen after the fourth gear.
I know. MAGIC.