There is also the question whether Jack really was incompetent. I have a feeling that Aguirre would have complained no matter how well Jack performed, it´s just the way he is.
Agreed. Jack can't have been *that* incompetent, otherwise Aguirre wouldn't have hired him again. Of course Aguirre complained, and in his eyes "ranch stiffs never did much of a job". That's just Aguirre being Aguirre.
But he understood that the 25% loss of the previous year hadn't been Jack's fault. That's why he hired
two guys the next year; "bad predator loss, nobody near looking after em at night". Jack couldn't watch the sheep 24 hours a day, he had to sleep, take on other chores, get supplies, etc.
As to why the boys applied for the job: they were hungry for any job, like Lee already said. Jack was also "crazy to be somewhere, anywhere else than LF".
I think the fact that the job came with lodging was the determining factor for both of them. Jack didn't care for the kind of work as long as it led him away from LF (and OMT!). And (movie)Ennis didn't have any place to stay. He had lived with his brother until the brother got married a month ago. "No more room for me. That's how come me to end up here."
... both high school dropout country boys with no prospects.... that's why they applied for the job. The world and the better jobs didn't exactly wait for them.