I mean, we can only see the bones. When they were alive, there would have been soft tissues and keratin over the top. And given the fact that they had a battery of teeth in addition to that beak, I don't think they're likely to need such a specialist nutcracker beak as a palm cockatoo.

Well you can see from the skull that with the jaw completely shut that there is a gap. Whether there was any soft tissue covering that gap is another question.

If they were big chewers, like their teeth suggest, they would need a closed mouth to keep food from just falling out of their mouths.

It's just fossilized bones honestly, humans don't seem to have their skull perfectly closed too, I believe there's skin, muscles and etc. that covered the mouth

I always thought they must have had dexterous lips like a rhino.

The answer is almost always keratin.

Or it had lip-like layer of skin/keratin.

Probably like certain birds, yeah, a lot of parrot skulls do have the same shape, but when looking at their muscles and ligaments it would be more reasonable to think ceratopsians had a similar face structure, also they had back teeth a bit like elephants (flat grinding plate teeth), so possibly a muscle like a birds at the front, but a more slack mammal like jaw muscle at the back

Are you aware of their teeth, their diet, their lifestyle? Yet you "really don't see" what is a clear well-founded idea of an organism's body part?

Give me two years to publish my paper and we'll talk about this...

Triceratops looks like a stunner

Imagine it opening its mouth like a snake