Magic/monsters and even dungeons can and do form and exist naturally in meshi's world, we know from the raspberry chapter that natural dungeons exist and natural monsters exist, its just the giant bloated demon dungeons that are unnatural (and mana is a naturally occuring energy source, even shit like lemons canonically have a naturally occurring amount of mana in them)
Demons are literally not of this world, they were brought through accidentally from a different dimension because of the ancient races trying to make a perpetual energy machine. It's just a straight up fact that they should not exist in this world, they're a blight/tumour on it that only serve to consume more and more until they kill everything through their 'hunger' (and they don't even need to eat in the first place, so its not even some sympathetic animal thing)> Another hint is that the demon has already been let loose on the world once before, yet it didn't end
Implication is that it got sealed before it actually reached the surface, we know that there is magic in this world that can be used to seal the demons into objects (like how the lion was sealed into the tomes), we even see Kabru telling Pattadol to set up such a spell when Marcille first becomes DM, but she wasn't able to do it in time. Presumably in the Utaya incident it was very similar to whats going on now, where monsters spilled onto the surface + the demon has almost done so as well, but it was stopped before it could.