> you can't leave cows unattended.
Of course you can. There are remote areas that essentially let cattle roam freely over thousands and thousands of acres and bring them in as little as once a year for maintenance (branding, medication, castration, selling, etc). No fences, no roads, rarely ever being seen aside from routine checks of water points (which is becoming less common with remote monitoring equipment).
> they wander off, get injured, fall ill, might be stolen, and all kinds of things. if they don't eat well enough they might not be worth slaughtering.
> you can either keep them in cages and attend to them there, with all the distasteful complications that brings, or herd them in large pastures at the cost of very difficult and dangerous skilled labor.
There is not much dangerous skilled labor at all. Difficult, yes like a lot of manual labor. Dangerous and skilled no outside of some rare cases like the use of helicopters to drive cattle in aforementioned remote areas with terrain sometimes not even accessible to motorbike so helicopter and horse is used. Importantly even in those situations, the amount of labor required per value of beef produced is simply not very high.
And those aren't really the options. Feedlots are increasingly used (what distasteful complications?) but most of the world's beef is not produced that way, and where those are used it is often as a finishing step after a free-range life to improve consistency and make up the last bit of weight.
There is very little that is difficult and dangerous about the labour required to herd cattle a couple of times a year, unless you are talking about the gyrocopter cowboys in far north queensland and the Northern Territory, of whom a better description would be crazy or insane
Feedlotting is more labour intensive, but then the beef is more expensive because in some markets it is considered of a higher grade, and they may weigh more
it will certainly require some training to attend to a bioreactor but assuming an eventual working model they seem much less problematic than a wandering animal, and would probably suffer less and get sick less than lots of caged animals.