B4: The Lost City is a classic module for D&D. At one point it (in)famously stops giving full description of the rooms but instead lists monsters in each area and tells the DM to figure out why they're here themselves. Once the reprint will show up in new anthology, I'm sure people who complain online whenever WotC uses "ruling not rules" or "DM decides" or "these parts were left for the DM to fill in" in their design (and then continues buying WotC books to keep bitching and doesn't touch 3rd party or other games for some reason) is going to be normal about it. /s
There is leaving space for the DM to inject some creativity, and then there's deciding that you don't need to actually produce a complete product because you know your customers will do it for you anyway.
I mean, it's not like any of the published rules are mandatory. Just because they're in a book doesn't mean you need to use them. But them being in the book means you don't need to come up with your own half-baked, undocumented, and inconsistent "rulings" if you don't want to.
And, frankly, it's not a symmetric situation. Published materials are suggestions that, ideally, are crafted by experts and well play-tested that may be ignored if chosen. Unpublished materials cannot be opted into.
To go one step further, it's not just that their customers will fill in the gaps, they'll also take the blame.
The DM struggles with a products that provides them with little to no support? "Skill issue. You just need a good DM."
The DM works doggedly to fill in and paper over the gaps? Sure, they'll appreciate the DM, but WotC gets some credit because "This module is so much fun!"
Also if its from the developers of the system you also, hopefully, can assume that they are keeping with their own visions and intentions which should be healthy for the system. Even if its just "Story" content, it'd be really weird to see a room full of random sci-fi crap in what has up until that point been mostly a gritty fantasy dungeon. Also, people can rail against this all they want, but people tend towards authority. The developers and publishers saying X feat or edge or whatever is useable in X setting, it doesn't leave a lot of room for inter-group bickering about it, but even then its opt-in.