Momentum of rear derailleur wheels?
The derailleur wheels are small and light weight, so have very small moment of angular inertia. The bearings they run on need to withstand a significant force due to the action of the chain, not just the weight of the wheel. So the bearings are fairly robust and lubricated. As a result there is a little "greasy sliding resistance friction" (for want of a better word) in the bearing. The resistance is small and its effect on riding resistance virtually imperceptible, but as the wheel's angular momentum is also very small, it spins down rapidly if spun by hand.

If the wheel was engineered like a fidgit spinner then it would spin for ages, but its bearings would fail within minutes.