Well, for my part I think it's pretty obvious why many people don't play high level D&D:
* Most games start at low levels, and so it takes too long to level up relative to how long people remain interested in a particular campaign/party/PC/playgroup;
* The PC builds get more complicated, especially spell-casters;
* Most GMs rely heavily on GM authority over the fiction to manage how play unfolds and to control the scenarios they are running, and at high level it becomes increasingly hard to maintain that authority - especially because of how high-level D&D spells interface with the fiction (in this thread, see both
@Minigiant's recent posts, and
@Zardnaar's post about "needing to be paid" for the effort of GMing high-level D&D);
* Various special cases of the previous point: fetch-quests for high-level PCs make less sense; dungeons for high-level PCs don't work as well (eg because walls are less constraining, the traps and creatures get more absurd, etc); high-level PCs are harder for the GM to "prompt" or "manipulate" through scenarios via threats and/or inducements, because of their greater capabilities and self-reliance; etc.
I think the relative lack of modules and monsters is perhaps a contributing factor, but also is an effect as much as a cause.
I don't think that the reason for not playing high level D&D is because all the mythic scenarios that might be played out at high level are being played by groups who have restatted all that sort of stuff for mid-level PCs.