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.
And combat encounter building is a core pillar of the game. It should not be a loosey goosey "rule of thumb". If anything, it should be the most reliable set of instructions in the book.
But some monsters are strong against certain builds and weak against others. Some monsters are stronger in certain environment and entirely nullified by others. Some monsters are stronger given certain allies and weaker when alone.
If you could devise a system to assign monster complexity based on every scenario you can imagine that monster being part of, then either that's an astonishingly small number of scenarios or an absurdly complex calculation to force on anyone.
Bullshit man. Pathfinder 2e had incredibly tight math behind it's design and very simple ways for dms to use it, dnd could easily do the same. Especially since dnd's direction seems to be about giving as little mechanical choice to the players as possible.