Why does NEC require circuit breaker locks only on hard-wired appliance circuits?
Why are lockable disconnects only required for hard-wired appliances?


Because if they plug in, you can just unplug them :) 

The purpose of disconnects, whether this type or those in fluorescent lights, is to allow you to unhook it when hardwiring.

The purpose is for lockout/tagout

This is why Code requires a disconnect switch within line-of-sight of the equipment in question. That is true in all instances; mainly this is applicable to industrial.   

If a line-of-sight disconnect is not practicable, then Code is allowing a lockout/tagout device in its stead.