I think that WotC probably believes that you can though. Would it be that hard, for example, to "just drop the Hell adventure in the middle of" Greyhawk City or Sharn or "Homebrewpolis"? Probably not. Besides, I believe that TheSword has indicated on multiple occasions that they mostly play adventure paths rather than more sandbox games.
I'm beginning to suspect that WotC does not build its adventure books strictly like you expect them to (i.e., Paizo). WotC very often uses these adventure books as stealth GM resource books for a variety of settings, themes, and campaign types (e.g., urban, naval, jungle, etc). WotC uses this as extra incentive to get people to pick up books that they may otherwise not have an interest in running. They may prefer running their own adventures, for example, but Adventure Book X has guidelines for running desert campaigns, so now they're more interested in buying it. I suspect that they figure that you can either run a "bog-standard, quasi-medieval...vanilla setting" on your own or that there are more than enough such adventures already out there. So I don't think that they are not driven so much by "the expectation of continual originality," but, rather, "the expectation of further consumer utility."
If you want an adventure with more "bog-standard" themes, encounters, and the like, then either you or WotC needs to think of a compelling selling point of utility other than a preference for "bog-standard" fantasy.
It's also worth considering that maybe 5e is moving on from being "bog-standard" as a result of the changing demographics of the wider playerbase. It may not be as interested in "bog-standard" fantasy as you are. If that's the case, then that's also not so much "the expectation of continual originality" either.