In the IETF's case, this is because, by policy, RFCs are immutable once published. You can update an RFC by publishing a new RFC that specifies the changes, or a new one that obsoletes the original entirely (this document does both to various RFCs). There is also an errata process for simple mistakes.
That means that minor changes tend to accumulate in other documents over the years. You could imagine tooling that automatically incorporates these changes into the presented document, like you suggest, but someone would have to build that tooling. That is a bit challenging, as the format of an RFC that updates another is not designed for such automated processing (e.g., go read this document and tell me what changes should be made to the text of RFC 5961, which it updates), and because the format of a published RFC is not semantic (e.g., adding a new section will not automatically renumber the remaining sections, because it's just a .txt file).
You could also design a different policy, but immutability has its advantages, too, and the current policy has been in use for decades, so the case for switching would have to be pretty compelling.
So for now the "Updated by:" header at the top of each document is what you get.
Technically all "update" RFCs could be "obsolete" RFCs - just copy the whole previous RFC and change the parts you wanted to change. The only advantage of "update" RFCs I can think of is that it tells the reader precisely what is being updated, but that can be done by just a changelog.
That means that minor changes tend to accumulate in other documents over the years. You could imagine tooling that automatically incorporates these changes into the presented document, like you suggest, but someone would have to build that tooling. That is a bit challenging, as the format of an RFC that updates another is not designed for such automated processing (e.g., go read this document and tell me what changes should be made to the text of RFC 5961, which it updates), and because the format of a published RFC is not semantic (e.g., adding a new section will not automatically renumber the remaining sections, because it's just a .txt file).
You could also design a different policy, but immutability has its advantages, too, and the current policy has been in use for decades, so the case for switching would have to be pretty compelling.
So for now the "Updated by:" header at the top of each document is what you get.