I think a lot of sci-fi fans (myself included) don't pay much attention to that because, well, it's not why we read sci-fi. People are boring. Cool science and weird technologies and interplanetary-scale politics are fun. For every story you'd like to tell to make a character "deep", there's a whole genre dealing with it in real-world settings already. Troubled teenagers? Sure, there's a section on Amazon just for that. People in love? Every other book is about this. Etc.
Just my 2¢.
I don't care how hard your SF is: it is always and without exception about what technology does to people--otherwise it's an engineering manual for something that doesn't exist. A lot of the "greats" of hard SF never got that. (Forward, again, comes to mind.) I have a more charitable opinion of Egan than the OP does, to be sure, but I have a strong reaction to the notion that the human element of this should ever, ever be played down. Everything, ultimately, is about people. We miss that at our peril.
This is why most of Heinlein (the tail end of his career notwithstanding--we all know it gets weird, let it lie) still holds up so well despite so much of technology and society having changed in the interim. Even books that weren't trying to be hard SF (his juveniles, which IMO are some of his weakest books, tended towards this) had engineering and mathematical verisimilitude, but it was a concern set well behind the characters who had stuff to do and learn and grow through. (Most of the time. Lookin' at you, Red Planet.)
Egan is good because nothing in his books is "whiz-bang". He's not writing about turboencabulators or laser guns; he's writing about worlds in which nature itself is fundamentally different, and in a much more fulfilling way than you would find in a fantasy book (because you know Egan's worlds are at least as mathematically viable as our own).
I disagree that SF exists to say something about people. Authors like Egan use it to say something about the universe itself, which is a much more vast and beautiful topic than the everyday goings-on of humans. Perhaps your focus is too anthropocentric to enjoy it.
I don't think you're appreciating how much other people's tastes differ from yours. I love to read an engineering manual for something that doesn't exist. That is exactly why I read Greg Egan books. The plot is a motivation to get me to think about physics, not vice versa.
For example, I've been reading Weber's "Safehold" series recently. Sure, characters are interesting. But I'm as much - if not more - interested in how the characters solve problems within the constraints of the environment they live in. I love to read about politics, I love to learn about naval and ground warfare strategy, about how development of technology transforms the society (not individuals).
I believe both kinds of preferences deserve to be satisfied. But the "people stuff" is done by 99% of the books out there. So I don't see lack of character-focus as a valid criticism of the remaining 1%.
That sounds incredibly boring to me. Count me as one sci-fi fan who doesn't think about it that way at all.
So whereas a when you read a western, you basically know everything about all the basics of the world, from the basic laws of physics through having a pretty decent idea of why "the Claytons would like to burn down the town" (despite having just those nine words, a genre-savvy Western reader has already got three or four basic plots in mind), and thus those things do not need to be explained, in science fiction you need at least some explanation of how the world is different.
Jamming that into a story pretty much has to come from somewhere else. Even in non-science fiction, seeing a novel perfectly balance Character, Plot, and Dialogue, to say nothing of the other elements of a story, is extremely unusual. I mean, like, award-winning unusual. Throw in the requirement that you have World Building in there and you'll generally have to pay the penalty somewhere else. It's just part of reading science fiction.
I can not recall if I first saw this argument from Niven or Asimov (or, worst case, one of the other classic writers); anyone who does please reference if you'd like. Unfortunately, my science fiction bookshelf is all meatspace and the indexing is terrible.
However, I would ask you to consider how many of those stories take place in a literally different shape of space-time.
Not just lip service to vaguely handwaved "strange space-times" in the style of Lovecraft, either, but extensively realized alternate geometries that drive the story.
There's a reason you don't get all of these things at once. At least, I can't think of a story that hits all these points at once.
Btw, I'm not expecting you to change your mind about Egan's work. Even by the standards of science fiction, I'd consider him a niche taste. If he's not to yours, party on and enjoy what you want to. I'm just trying to explain his work.
> and even their ideas are mediocre
I don't recognise Greg Egan's work from this description.
Cryptonomicon, Anathem, and The Baroque Cycle are anything but.
For Vinge, I recommend Marooned in Real Time. To me, that is his best novel.
I'll check out Diamond Age, however, as I'm a fan of nanotech and am interested in how far scifi can push it.