cus we are in an era where 8 billion ppl are roaming around, human resources wouldn't be as sacred. Supply and demand also apply for jobs too ya know.
It's also indicated that the area where MC is living is filled with sick ppl, so herbs on the rise ? And he probably clears a considerable amount...
He has a very high level in those herbs-gathering-related skills truth be told. Also, there are actual risks like monsters so maybe low taxes for these risky jobs ?