The more time they spend in a truck, the higher the risk of "transportation sickness," which is basically animals being stressed out from being in a weird truck going down the highway and being more susceptible to a bunch of stuff. Interesting aside, that's part of what made Roosevelt's ranch more successful, he was supposedly one of the first ranchers to stop transporting live cattle to Chicago for sale, and instead slaughtered on-site and made use of the newly invented refrigerated boxcar.
There's money in direct sales, of course, but you have to a) be inspected for general sales and b) use a butcher who is inspected and licensed for i) in-state and ii) out-of-state sales. If you ever buy a "share" of an animal and pay to have it butchered, you may notice the meat comes in packaging with prominent NOT FOR RESALE markings -- that's why, it's a local butcher not USDA licensed for general public sales.
Buying "shares" is a skirt around the law, but most people don't want to deal with the amount of meat from even 1/4 a carcass, and they're usually hard customers for local butchers ("Your butcher is robbing us! They wouldn't give us
T-bones and NY Strip!"). Supposedly if you're found to be doing too much of it, or "automating" it too much for the "shareholders," the USDA will come down on you for it (IMO rightly).
You also then have to market the meat. My wife and I had offered to help set up a canned site to help with it, but my parents were too concerned about the licensing requirements and that it'd be throwing more money into the hole that is subsistence farming!