They aren't interesting because origin stories are rarely something the character did, and often something that happened to them. They are passive, not active. Take two of the earliest origins in comics-Superman-Krypton explodes, baby Kal is sent to Earth, found and raised by Kents. Foundational sure-but doesn't tell you anything at all about who Clark/Kal is because Kal does nothing in the story, it happens to him, he isn't really a protagonist in it-Jor-El, the Kents sure, Clark/Kal, not so much. The protagonist faces conflict, overcomes it, and grows and changes as a result. That is the core essence of story. The origin story of Superman is not Superman's story. Therefore it is the least interesting story about him. The same with Batman/Bruce Wayne-he saw his parents killed in an alley and swore vengeance. Great. Foundational absolutely. Reveal anything about who Bruce/Batman is, how they will do that, or even whether that was an empty promise in the moment as so many reactions to grief are? Absolutely not. Those are the things that make Bruce interesting-what he does, not what is done to him. Bruce as actor, not passive recipient. The origin is a MacGuffin that lets you get to the real story being told about the character, where the character does story things and is not a passive recipient of the action merely reacting to events. So I agree, they are foundational, but that makes them no more interesting because of it.
the 11 page story of Spidey's origin in Amazing Fantasy 15 or the 8 page origin of Dr. Strange in Strange Tales #115 are good origin stories, they set the foundation but tell stories of the character doing something not having something done to them and becoming a super-hero as a result. But those 11 and 8 pages are absolutely enough. They don't need to expanded or decompressed into a mini series or arc. They do what they need to do so you can go on and tell more interesting stories about those characters. I find the need to go back and continually retell or fiddle with origins stories expanding them is more a sign of a lack of other interesting stories to tell by the storyteller than an effective way to increase interest in the character. If you want to be remembered for creating a character, don't rest on your laurels of some origin story, keep telling dynamic, interesting stories with that character. Ditko's legacy with Doc and Spidey is more about the other great stories he told about them than the origin stories at the beginning. Comic creators are story tellers, their legacy comes form continuing to tell interesting stories, not be one-hit wonders with an origin story.
When I was a kid and discovered Spidey, it was because of the stories being told about him. I got to read his origin in the big Origins of Marvel Comics book my cousin had, and it was cool, but not as cool as they stories I got to read every month where Peter juggled his day to day life with bring Spidey, his hunt for the villain of the week with paying his bills, making time for Gwen, etc. etc. (I discovered Spidey in the 70s and got Marvel Tales w/SA reprints more often than the current book). What made Spidey interesting was the stories being told about him that revealed his character, not an origin story told a decade or more previously, and it was Spidey's longevity and the oeuvre of stories about him that cemented Ditko's Spidey legacy, not an 11 page origin story in Amazing Fantasy 15.
If you have interesting stories to tell about a character that take place at different points in their career, those can be good, but it has to be because they are good stories in and off themselves and not because they are expanding/revising/revisiting continuity. Kurt Busiek's Untold Tales of Spider-Man is a great example of that. They took place in the Ditko era, but were good stories in and of themselves. They didn't need to be told because Ditko didn't flesh out the character and they didn't cause anyone to forget Ditko role in Spidey's creation (there were a whole of of other things that did that but that's a different conversation). The only reason they needed to be told was that they were good stories that interested readers and this sustained and built the legacy not diminished it. But if all you do is mess around with origin stories, you never get to do that. And if the origin is the most interesting thing about the character and that's the best story with them, you are not going to sustain readership. If each story is less interesting than the first, people aren't going to keep coming back, and that origin story, character, and legacy will be forgotten.
-M