Parallelismen is not cheap, in the end it costs the same. It is one resource for a long time vs many resources for a short time.
Humans are a lot more expensive, it takes ~a year to find, interview, hire, onboard and offboard an employee, while an agent can be spun up for 10 seconds, do its work and vanish back into the ether.
Agents are lambda functions where humans are bare-metal servers that you have to rack yourself.
You might want to make a separate point about the cost difference of getting more done by hiring more people or running more agents, but that is a different point.