That's not the whole story. "The dog swam across the ocean." is a grammatically valid sentence with correct word order. But you probably wouldn't write it because you have a concept of what a dog actually is and know its physiological limitations make the sentence ridiculous.

The LLMs don't have those kind of smarts. They just blindly mirror what we do. Since humans generally don't put those specific words together, the LLMs avoid it too, based solely on probability. If lots of people started making bold claims about oceanfaring canids (e.g. as a joke), then the LLMs would absolutely jump onboard with no critical thinking of their own.