why do people think cycling is for bicycles Only?

Bicycling means two wheels, cycling is to repeat over and over. Cycling is the act of ridding a bike. Motorcycling is the act of ridding a motorbike.
 
There's a separate category for motorbikes.

This section is for human-powered machines that have wheels. They can have more than two wheels, though - bi means two. Tricycles, etc, are fine. However, unicycles are more of a performance machine, so I'd go elsewhere for them.
 
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