How is the writing in "The Road" by Cormac McCarthy in some ways more like poetry than narrative prose?
Cormac McCarthy has an unmistakable prose style. There is no grammar and McCarthy jumps in and out from dream to reality in "The Road".
The reasons that you stated above in your question partially answer the question itself; McCarthy has a very unique feel to his writing, and he does jump in and out of dreaming and reality within a passage. Poetry tends to do that much more than narrative prose. One of the main purposes of narratives is to tell story; the emphasis is on plot. Poetry is more about mood and feeling, and McCarthy's storytelling has a definite mood to it, so in that sense, it is very poetic.
Adding to its poetic tone is his unique grammar. Poetry takes liberties with grammar, using the language, words, and grammar for impact, rather than for understandability. Take for example the opening sentence: "When he woke in the woods in the dark and the cold of the night he'd reach out to touch the child sleeping beside him." Although not necessarily incorrect in grammar, it is a lengthy sentence with a lot of "ands" that makes it seem like a long, drawn-out breath. It is exhausting to read, which is probably how the father feels, exhausted at his quest for survival, and in his worry for his son. So, McCarthy uses the grammar and the sentence structures, to convey the sense of never-ending exhaustion that the travellers must feel. That is a definite poetic angle there.
Another poetic angle seen is his use of those long sentences, mixed with incomplete sentences (poetry often uses incomplete sentences) thrown in with tiny sentence fragments that up an entire line. For example, "I'm right here," and "I know," take up their own lines. This emphasizes their importance, how being there for each other is important, and, being isolated on separate lines like that is a poetic structure. He also uses a lack of correct dialogue quotations. When they speak, he doesn't set it off with "Are you okay?" the father asked the son. Instead, he just throws it in as part of the line, not setting it off with quotes. Poetry doesn't use direct dialogue very often, so McCarthy's text models poetry in that sense. It gives the entire thing a more passive feel, a more dreamlilke quality. McCarthy also uses a lot of poetic techniques-imagery, similes, metaphors. Consider the following line:
"Then they set out along the blacktop in teh gun-metal light, shuffling through the ash, each the other's world's entire."
He uses imagery and metaphor to describe the light, and concise langauge to describe how much they mean to each other. That is more like a poem than narrative.
So, through weaving reality and dreams, using distinct sentence structures and grammar, the lack of dialogue markations, and poetic techniques, McCarthy's narrative is very poetic, giving it all a very distinct voice.