There are good reasons for it. The main one is that it's for the audience, but another common reason is that humans are usually the most numerous race in these worlds. Because of that, it's rare for other races not to have seen humans, while the opposite isn’t true. In the few stories I have...
Nobles are above commoners but they certainly aren't above royalty. The manager of the territory is married to the princess and the territory belongs to a prince. Even if said manager is a commoner his connections put him far above lower nobility.
Looking back at around chapter 34 he was granted...
It would be terrible in a machine shop, but it doesn't really matter if the product covers the costs. For advanced computer chips for example it has been common to have a large amount of lower quality chips. These chips are then are reused in lower end hardware.