[neatoman]

I think you're missing the point here, it's not meant to be practical, it's meant to be visually striking.

They're warlike brutes who went so far as to destroy their home, now they're clinging onto the remains. The image of conelike asteroids with domes attached is fairly unique and memorable. More importantly, it serves as a reminder of who the Triceratons are as a people. You are right about how it makes very little sense, they would be better off trying to find a new homeworld. And yes, you could in theory have their home be a living planet. However, neither is to the point.

It would be like questioning Godzilla's origin, anatomy and behaviour. No, radioactive waste won't mutate an ancient life-form, a creature that size wouldn't be able to stand, it wouldn't breath fire and destroy cities. It's just meant to be a spectacle. If we saw a living world, that would be less interesting to watch. If you really want that, why not have a prequel or a story in which they colonise a world?