Should developers be paid by the lines of code they produce?
The reason not to do things like that is if said metrics can do more harm than good. However freight is pretty commodity by definition. Aside from not doing damage to goods and people, throughput is basically the only things that matter in that context.
Employees demanding a "standby base rate" would also be sensible if they were required to wait while lacking work to do.
Why can't what developers produce be commodified?
Regulated taxi fare formulas usually include both time and distance. A mile in a traffic jam will be based on time. Ten miles with no traffic will be based on distance. Why should truckers not get paid for sitting in a traffic jam if cabbies do? It still costs the trucker time that he can't be doing/working something/somewhere else.