Sight reading gets your whole brain activated.
When I used to tutor standardized test takers, one skill we worked on was active reading during the long reading comprehension sections. Active reading simply meant asking questions as you read: “What does the author mean?” “What can I infer?” What is she going to say next?” “Why did she include that detail?” “How can I contextualize this statement?”
Great sight reading is about using the same sort of active listening.
“How do I find my note for my next entrance?”
“How does my part interact with the parts around me?”
“What are some pitfalls to sight reading this spot?”
“How do I interpret this phrase? Can I infer things beyond the markings?”
“Where will I breathe?”
Not to mention, “Was that the right note?” and “Was that the right rhythm?”
Phew, that kind of active listening takes work. It takes the whole brain.