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
I think it's mostly cowardice, personally. People don't want to risk putting their own choices into a game based entirely on choices, just in case they aren't as good. It's better to use someone else's decisions than risk your own pride.
Then you have ignorance. A lot of people don't know how to fill the gaps, and WotC has never bothered teaching them how. Any rules they did get are rules of thumb and aren't something to use without thought (like CR), so people complain for reason 1 again.
I disagree. DM's always have the ability to put in their own choices and, in this case, room descriptions, regardless of what a module says. But that is work, and one of the things you buy a module for.
To make an extreme example, imagine I sold a campaign module called Blank Slate, where every page just says "and then you decide what happens next" and "decide what rooms are in this dungeon and what monsters are there."