A key difference seems to be that in show business you get many more offers. In software you might only switch jobs every few years. So the benefit of saving X hours every time gets amortized to a very low amount.

No, in show business you get lots of offers at a higher level but at a lower level the agent is also in charge of going out and selling you, also at the higher levels if you hear of a role you want you tell your agent, Sy, get me that part.

I know of one guy who was a consultant who had a dedicated agent to handle this stuff for him. Which lots of consultants get their jobs through agencies who match consultants with companies anyway, so it just seemed like a natural jump up to this level for him.