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frapco IMO it's cuz Dad thinks they're sending him on useless errands (remember, he thought the Immortal House, or whatever it's called, was a bandit hideout) that happen to have some sort of danger involved. He honestly thinks the Murim folk are all super OP, so he doesn't realize that these guys he's fighting are actually strong - he believes they're fodder, or bandits, or weirdos, and he's more or less had enough of it, even if the pay is "good" (relatively speaking) - he just wants to see his kids, man!
At least, that's my take on it