Why do motorcycles use a chain drive instead of a gear drive?

Those who have studied machinery know that transmission includes gear transmission, chain transmission, shaft transmission, belt transmission (including timing belt), etc.Each transmission method has advantages and disadvantages in terms of cost, durability, environmental requirements, accurate transmission ratio, transmission distance, easy maintenance, noise, weight, transmission efficiency, etc.The characteristics of chain drive are low cost, general durability, accurate transmission ratio, long transmission distance, high noise, light weight, general transmission efficiency, low environmental requirements, and easy maintenance.The above features are suitable for motorcycles.If you use gear transmission, it is not impossible, but it is not the most suitable.It is difficult to match in terms of cost, environmental requirements, transmission distance, weight, etc.