I'll tentatively agree with the OP.
Because the epic rules are core, one would expect them to be the basic assumption, and thus predict campaigns to go into them, even as they project that campaigns go into the 15th or 10th level. This means that the core rules should be designed with the epic rules in mind, that the core campaign shold assume the use of the epic rules, and that the things that are the most powerful in D&D (like the demon lords) should make good use of those rules.
That they don't is a tragedy. It's like designing fire characters that didn't make use of the game's rules for subtypes and energy types. You have a usable tool for depicting extremely powerful, world-shaping, beyond-mortal powers. Those rules have an appropriate use. It's not an entirely common use, but to ignore a rule just because it's not a common circumstance...I mean, how often does the turning radius for flying creatures of manueverability (good) come up? But the rules are there, and they should either be used for what they are designed for or redesigned so that they are useful, but not ignored.
That many people don't play higher levels is an effect rather than a cause.