See, I think RPG books are meant to be read and enjoyed. That's always been how I consumed them. If they did what you propose, people would ignore the prose because they need the rules to play, and soon enough folks would be clamoring for them to be sold separately. When the rules sales outstrip the prose, WotC would conclude its not worth producing and discontinue it.Game books are manuals. They need to be referenced as such.
If it were up to me, each D&D rulebook or adventure would come in a slipcase. There would be a big robust hardcover with all that juicy, lovely prose, but the other book would be a reference manual that had far more in common with a complex board game rulebook.
Just an aside so it's clear where I'm coming from: I am an RPG freelance writer that gets paid by the word. I get why RPG books look the way they do. But thay are by and large terrible for reference. Runing a Paizo AP or big WotC 5E adventure is a nightmare of preparation. Important details are buried in walls of text and no one gives any thought to infographic design.