Venat specifically mentions the sharks as being made because it became trendy, not because of the world actually needing them. Others are actively making the world a better place and creating things that actively fit in the world as a whole like it was intended. Others like one Ancient in a FATE sends us against a bunch of strong creations because he thinks we're a disposable familiar that happens to be strong and he wants to make something strong with no other purpose given. The only reason why you would need to create monsters to fill your ecosystem is because you've created monsters already and it becomes a sort of arms race in order to keep everything balanced. Elpis is also dangerous enough where it's been mentioned that Ancients have been injured or killed so it's not all sunshine and roses and then Pandaemonium, which is a league of its own.
Either way, I thought Elpis was much stronger on its characterizations of Ancients when it stepped away from the monster creating stuff, which ends up cheapening a lot of the pre-existing world anyway since it turns out a lot of the monsters we've encountered in the world and even possibly the Lupin just boil down to "a bored Ancient made it" instead of simply being a fantasy world full of monsters and it takes a bit of the sparkle out when stuff like this ends up getting explained.
The scariest thing about the Ancients overall that we learn later on in that whole arc isn't that they make this stuff, it's that they individually feel like they have the power and the right to make decisions for the whole planet and they they know best and that's how you get people like Hermes, Venat, and Emet-Selch.
I'd like to think the writers are better than that considering all the destruction we've left behind that the existence of the Leatherworkers' Guild. I just figured it was there to introduce us to the concept of reducing creatures into aether, which comes up later, and to give us a reason to kill X amount of things.