I find your disdain for that feature to be amusing, because one of the first plugins I've downloaded for Firefox, SeaMonkey, and Pale Moon whenever I've installed them on a computer is an EPUB reader. As much as my genuine preference would be to have a web-browser with a "Do One Thing Well" mindset, the fact is that EPUB is great because it's just a compressed and ordered set of XML files, which a web browser is very much built to read. I don't see a need for Calibre when either my web browser or Sumatra PDF can pick up and display my EPUB books properly.
So for me, the fact that I don't need an addon to do that would definitely have sent me to using Edge over Chrome or current Firefox, if I were on an OS that included it.
My disdain for that feature comes from a position of someone who reads e-books a lot. A browser is about the most useless place I can think of putting it, because it is very rare that I want to just take a peek at an ePub file while I'm surfing (unlike with PDFs, which serve as a de-facto pixel-perfect text format for the web). A proper book reader app needs e.g. library management.
Ah, see, there's the big difference. You and I have vastly different use-cases - something I've noticed seems to get disregarded lately in dev circles. I don't need a software library manager, I already have a file manager (Windows Explorer) for that, and I really can't help but read to the end of a chapter. So all I need is the XML parser and renderer. There is no such thing as 'one true way' to use a computer. I don't ask you to use my solutions to the problems you face; I just ask that if you develop one of the programs I rely on for that solution, that you don't go and break it.
Over time, I've found that (in general) Microsoft is less prone to breaking what has already worked than Mozilla or Google. I don't know about Apple beyond iOS, and I don't know about that for the backend, because on the iPad, the only non-OS Apple software I use is iBooks (hasn't changed much in terms of 'tap a book, read a book'), Notes (hasn't changed much in terms of 'open a note, type a note, it autosaves'), and Podcasts, which also hasn't undergone much change. Heck, it's the App Store itself that's most broken for me now, to the point that I've come to rely on sites like appapp.io when I want to find a program or game; and then I pop over directly to the app store page for the software. With Google, I'm glad I never used GMail in the first place because I've heard the redesign is terrible. Maps are still pretty good though there's been some new "features" that made my life more complex; but every new version of Android that I get when I change mobile devices seems to bring with it a new host of settings issues and things it doesn't allow me to do without rooting it. Mozilla, well, let's just say the downturn started when Australis was mentioned and there have been no significant bright spots (Pocket, the introduction of telemetry, & Mr. Robot being significant lows).
Sorry for going a bit sideways to the topic, but honestly, TL;DR: different strokes for different folks. I hope you genuinely like your desktop ebook reader of choice so much that you wouldn't feel a web browser can do the job as well, but I don't need the heavy-duty support of a separate reader program when the addon or built-in feature works for my needs.
I don't have any objections to the OS having ebook reader out of the box - I just think that it would be better as a separate app, rather than a browser. Browser is for, well, browsing the web. Reader is for working with ePub files, for starters - even in the scenario which you describe. And it can still be a basic viewer app that just opens on double click, with advanced functionality (like a library) available for those who need it.
To me, it's the decision to shove that functionality into Edge that feels like a typical developer shortcut: they wanted ePub, and ePub is basically HTML, and Edge already does HTML, so let's do a reader as an extension! But it doesn't really make any sense to me UX-wise. Again, the only reason for something to be in the browser like that is if you routinely open documents of that type via links. And I just don't see people doing that with ePub - note how your scenario doesn't involve anything of a kind.
So for me, the fact that I don't need an addon to do that would definitely have sent me to using Edge over Chrome or current Firefox, if I were on an OS that included it.