The bodies of eighth-tier creatures are still quite easy to obtain; not to mention anything else, there are some right here in this forest.
In fact, if the forest did not contain powerful creatures, it would have been impossible to preserve it. It would have long been developed by surrounding cities as a source of timber. But with powerful creatures here and its distance from the city, it has been preserved.
After all, no matter how powerful a Magical Beast is, it cannot build cities, so it's not a threat.
Moreover, they do not want to fight powerful Magical Beasts. Because once they do, what if they accidentally get injured? The strong ones from nearby cities won't just stand by and watch; it would be more trouble than it's worth.
Also, once they make a move, others might show up immediately, leaving the praying mantis stalking the cicada, unaware of the oriole behind. It's precisely these kinds of concerns that have kept this forest intact.
