Well, that was my feeling too. This thing about business vs job is overly simplistic.
We are in service businesses and you can't compare that to something like a retail store or a professional practice. These all generate repeat business that is predictable. We don't normally have much inventory and our business comes mostly from new customers. Not every small contractor is going to be able to have an exclusive relationship with GC's, property owners etc. so that there is predictable source of work.
Still, I think all things being equal that a contracting business that generates say $1 million a year is worth the same as any other type of business generating that amount.
In the eye of a buyer however that may not be the case and that's where the discrepancy lies. It's not whether you run it as a business or a job but how attractive you make it to potential buyers.
Take two used cars, both same make, model and year. One has 150K on the odometer, other 30K. The one with high mileage needs tires, is dirty and could use some mechanical work. The low mileage one is clean and runs well. Which one is going to fetch the higher price?
So, I think the topic here should be "what makes a business attractive to a buyer".