Because back in the day people were okay with "level" meaning multiple things:
- character level
- spell level
- dungeon level
- encounter level
As per the AD&D DMG:
View attachment 121735
Notice level is never used by itself, but always in context to something else.
Sure, you could call it something else, but then other people would wonder why not just use the term "level" or something. More terms would make the game confusing to those people.
The problem is that character, dungeon and encounter level
could match in meaning if you want to, but spells don't.
We can talk about a "level 3" encounter for a party of 3rd-level PCs if we want. In the current edition there is no such thing as encounter level
formally, but it is only a matter of setting a couple of
defaults such as number of PCs e.g. 4 and degree of difficulty e.g. "medium". Then, we could say that a "level 3" encounter is one that has a total worth of 600xp. The 5e approach is significantly more nuanced than a single level value.
For a dungeon, if we really wanted to, we could decide to design it specifically so that all encounters on the 3rd "dungeon level" would be in fact "level 3 encounters". How many you would put there would depend if you want each floor to be doable with a whole day of resources (then you would put 6-8), or something else. Naturally, I don't expect many DMs out there being interested in such rigid dungeon design, but it can be done.
So a dungeon's 3rd level is an area that contains 3rd level encounters which in turn are "medium" (or whatever) difficulty encounters for an average party of four 3rd level characters.
OTOH
spell levels are whatever the rules system gives you. 3e and 5e grant the 1st spell level at 1st character level, and additional spell levels every 2 character level, because that's just what the designers felt right. I think it would be very silly to try and change the whole spellcasting system of a game just to match spell levels with character levels. It would be a lot easier and better to change the term for spells.
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Personally I always use "level" for monsters as a synonym of "challenge rating" for individual monsters, and I don't know why they do not in the books... the meaning of CR in 3e and 5e was always directly tied to the PC character level. If you tell your players "this
monster is 3rd level (CR)" then they know exactly that (in 5e) it means that a single PC of 3rd level should be typically capable of fighting the monster with the abilities available at her current level.
Then obviously, if I have a party of 4 PCs I can say "this
encounter is 3rd level" if it's made of four CR3 monsters. Things are less and less obvious the more you drift from a group of monsters with equal CR, but at least there's a baseline.
"Dungeon levels" on the other hand, I've never personally intended them with any particular meaning, and in fact I think I usually call them just "dungeon
floors".
Sometimes with beginner players I have talked about "spells
powers" (as in "powers of 10") rather than levels to avoid confusion. It doesn't sound very good in English but sounds better in other languages.