It ultimately boils down to commercial viability. Take the Matala theme, for example. It’s a work of art, but only has 2,000+ active installs.
Many theme users want to use something to make “their own” and there’s no way to do that with a theme like Matala without heavily customizing it. Ironically, losing the artistic value that was so appealing in the first place.
Then again, “just an arrangement of blocks. Usually grey,” sounds totally soulless and not very commercially viable either. I get what you’re saying, and don’t think this is the same as simply being “flat” or “minimalist.”
I believe the best themes are the ones that largely get out of the way design-wise, letting the user “customize” it naturally through their own content publishing.
This quote comes to mind: “Good design is obvious. Great design is transparent.”
It is difficult to balance great WordPress theme design with minimalism and customizability, but a few rare theme shops do pull it off with some consistency. Array, DevPress, Pro Theme Design, Elmastudio, The Theme Foundry, StudioPress, Foxland…just to name a few.