Farmers aren't salespeople. They often don't want to deal with the hassle of selling a part here, a part there. You want a tenderloin? What is the farmer supposed to do with the rest of it?

Far easier, more predictable and less wasteful of the animals to sell the entire lot to processors, who then deal with butchering, packaging and distributing individual cuts for them.

I could do one off short term programming contracts, but my customers would definitely pay much more for the overhead of me making a living that way vs simply getting a salaried job.