Why can't optional modules be referenced in future products?
It'd be super easy to have the "core" book be a Rules Cyclopedia with all the rules for how to play, four classes & races, DM rules, and even some monsters. Then you could then have a PHB & DMG that omit the basic rules but uses that space to cram in extra modules and options.
It'd be simple to have a slightly different trade dress on the PHB and DMG signifying they're "Standard", such as a coloured band at the bottom, and have future accessories do the same. And then they could release a third line that has the "Advanced" options a little while after that build and expand on the Standard game.
Basic accessories would be compatible with all versions of the game and wouldn't use any rules modules. This would include the Monster Manual and *most* adventures. Meanwhile, other accessories would be Standard or Advanced and list the rules modules they use on the back along with source. Standard adventures and accessories would assume a skill system (but not necessarily which skill system) and other assumptions. Advanced adventures might vary dramatically based on the modules used.
Because then a good portion of later releases will contain rules which are only used by part of the playerbase, the one who use optional rule X or Y, and the value all other players get from a book goes down. Also, having to account for every optional module and combination gets very complicated fast and takes up space. Just imagine a adventure where a challenge has to list several sets of rules just to account for different optional modules (Check to open the door basic, skills, skill point system, etc...). And just think of NPC
Having a basic and advanced set of rules would split the playerbase and increase cost for WotC without much gain.