While I will not argue with your general point -- I also don't really need highlighting and I read a lot of plaintext code -- I wonder about this.
Would this make languages easier for non-native speakers? Would improve comprehension?
It's funny that the industry spends so much time on syntax highlighting for programming languages, when humanity's written languages are arguably more complex and difficult to parse and master.
When I've been trying to learn languages, I can typically part-of-speech tag unknown words quite easily (common prefixes/suffixes/word length/sentence position give lots of information – and some of this is shared across languages as well). The comprehension difficulty is nearly always due to content words I haven't seen before (or have forgotten).
Somehow I never found a need to change that. I highlight comments, keywords, and strings. Comment and string highlights are helpful if they contain code-like text, to make them obviously not-code. Keywords give some structure to the text.
Everything else is frivolous to me. Books do not highlight verbs in green, either.