to "Ride lightly in our Garden of Eden"
The chain is enclosed protecting you from grease and protecting the chain and other internal parts from external debris.
Not readily seen but with one located in the bottom bracket, and the other in the rear wheel hub, are two torque control cans, that have a master slave configuration. These help permit this remarkable achievement in mechanical engineering.
A key element of the bicycle is the bicycle chain. This mass produced, taken for granted part of the bicycle makes the bicycle as efficient as it is. Close to 100% of the power is transferred from the rider to the rear wheel, via the chain. No other method matches this quality.