Why don’t two-wheelers have a reverse gear?

A regular motorcycle has a ‘sequential’ manual transmission, meaning you can only shift up or down one gear at a time. It is also designed to spin one way only. Adding reverse would require the crankshaft to spin backwards, or a much larger, heavier, expensive transmission. So it is easiest to dismount and push the bike backwards.

Regular motorcycle, Sequential manual transmission, Gears, Designed to spin one way, Crankshaft, Spin backwards, Expensive transmission, Push the bike backwards