hawkeyefan
Legend
Because it isn't defined as a number in the setting, just in the game as an abstract.
Neither are the characters’ stats, skills, AC, hit points, attack bonuses… and so on. But they’re there for the players to understand how their attributes relate to others.
But hard numbers do? Where is the PC getting those numbers from?
The PC isn't getting them. The player is. The PC doesn’t think “this thing has an AC 14” or “this thing has 90 hit points”.
Sometimes yes, sometimes no; not all monsters are met at their lairs.
But consider this:
Pre-set scene: mid-dish level party are walking through an unexplored forest en route to a castle or similar they know to still be at least a few miles away.
DM: "As you break out of the trees and are able to see across a clear shallow valley ahead, you notice a strange-looking creature about 400 feet away from you, just on your side of a creek that runs down the valley. If the creature were a lot smaller it would be similar in many ways to a St Bernard dog, but from here it looks to be about 8 feet high at the shoulder and probably has a few more legs than a typical dog would - looks like six instead of four. Safe to say none of you have ever seen anything quite like it. It was drinking from the creek, but as you appear it hears you and looks at you, then its mouth opens (and those teeth are huge!) and it charges toward you at high speed. It'll probably only take a couple of rounds to get to you, given how fast it's moving. What do you do?"
At this point, what's the justification for giving the players any further info as to the giant dog's capabilities, AC, intentions, or anything else?
Because you’ve given them very little that relays this information. Far less than the characters would glean from simply seeing it.
Which raises a point: should the GM be mostly communicating in character-speak (i.e. as a narrator describing what the characters see-hear-smell-etc.) or in game-speak (i.e. inserting game-mechanical numbers into that narration)?
Me, I far prefer the former.
As I mentioned earlier, I look at it as a translation. The characters perceive X, and the players are told Y. X is going to be descriptive, in-world information. Y is going to be game mechanics. I don’t see any good reason to limit either.
What I would not do, and don't think i have ever done, is say "The bandit captain looks like she is CR4/3rd level/4+ HD".
But you’d certainly tailor the description based on comparative CRs and Level, I expect. So the CR 4 NPC may be described as very dangerous if the PC is level 1, or relatively harmless if the PC is level 14.
Which just shows… these numbers are representative of things that are observable in their world. Telling the players the numbers is similar to the characters observing their surroundings.