A human doing the same is investing effort on their end. That means they will be somewhat selective about who they call, and are unlikely to call 30 businesses. Maybe they'll call 6.

The actual sweat-effort that the caller puts in is the evidence that they aren't being frivolous with the time of the call. When that goes away, and the cost to the caller of making 30 calls is the same as making a single call, then it quickly becomes a waste of time for the business, rather than a valuable opportunity worth pursuing.