That's… not the point of RFC 2119. The point is to specify specific words used to indicate various levels of adherence to a standard. Maybe "must" clearly indicates a strong requirement to you (as opposed to an optional requirement). Does "should"? How about "shall"? The English interpretation of these words contains shades of grey, and different people will interpret them differently, sometimes even depending on context.
Why invite such ambiguity? It leads to problems, when the designer writes "shall" and the implementer takes it as a suggestion, or the designer writes "should" and the implementer sinks months trying to satisfy what turns out to be a requirement the implementer intended as optional in light of unknowns.
Clarifying terms eases communication. RFC 2119 isn't about defining basic words of the English language for imbeciles. It's about selecting words to indicate precise levels of compliance. They could have equally named the levels P0, P1, and P2. Instead they chose MUST, SHOULD, and MAY to indicate the same, while reading more easily as part of a sentence.