Bicycles need service.
Parts get dirty and wear out.
Dirt gets in the chain and wears between the pins and plates, so the chain gets longer. As the chain gets longer it wears the gears. It’s much cheaper to replace a worn chain sooner rather than worn gears later.
Brake pads wear down.
Tires wear down.
Wheels get knocked out of true. Spoke tension gets uneven. Loose spokes will break. An untrue wheel will interfere with braking. A wheel with uneven tension won’t hold true.
Cables get dirty. The brakes open slowly. The shifting gets slow.
The derailer hanger gets bent and shifting becomes imprecise.