You must not see the value in dictionaries, or explanatory text, or examples at all.
Let me rephrase: would you have a problem with RFC 2119 if it defined the terms P0, P1, and P2? If so, why? (That would be a bizarre stance to me.) If not, why do you see it as problematic that they chose the words MUST, SHOULD, and MAY, instead of P0, P1, and P2? After all, there's no logical fault in choosing more memorable names.
I see this problem in code often too. Some people think it's sufficient to give functions and variables English names with "obvious" meanings. Yet however "obvious" the meaning is to you, it can be interpreted differently by different people because English is ambiguous. Such symbol names without comments ascribing them a meaning plain English might as well just be "xyzzy", "qwert", and "l33t". English-named symbols are mnemonic devices, nothing more.
Example: the code base I work on as two commonly-used functions, "findObject" and "getObject". One returns a reference to an object only if already cached; the other fetches it from disk if necessary. Which does which? Does "find" mean to go out and actively "find" the object? Or is it referring to the fact that it only returns the object if it is "found" in cache? After all "get" is a pretty active verb. I guarantee you, whoever named these (and didn't document them!) thought like you, that the meanings were "obvious" and "unambiguous". And I bet they were, to that person, at that time, in that context.
I for one, can never remember whether SHALL is mandatory or not, or whether SHOULD indicates nice-to-have-but-not-mandatory, or purely optional. They're terrible English words to use in a formal specification. But the mere fact that RFC 2119 disambiguates them as formal terms in the context of specifications makes them eminently usable for that purpose.