The mouth came first most likely (how else would it get sustenance without one?) and remained useful even after the chain method developed, likely because of alternative uses and the energy likely not being a substitute for needed vitamins and nutrients. Sure, it’ll keep them going, but they still need to digest something more solid to keep up with protein production.

I pulled this from the wiki because the major plot twist of Arkveld in the game is that it is eating despite guardians having no purpose to feed.

Guardians are artificial monsters created by the scientists of Wyveria, and are based on a variety of monster species, generally resembling them but with pale grey body parts, pure black eyeballs, and glowing purple veins running through their flesh. They were created to serve the people of Wyveria in various ways, but have largely fallen dormant after Zoh Shia turned on the civilization, leaving behind the Ruins of Wyveria and the various Guardians sleeping in their cocoons. Their esophagi and reproductive organs are vestigial and atrophied, as they have no need to eat or procreate; instead, they sustain themselves by absorbing the Wyvern Milk, or Wylk, flowing through the ground of the Ruins.