Isn't the iPhone/iPad supposed to be exactly that?
A single device, a single ecosystem, a single curator.. and outside of a vocal minority, millions of people love it for exactly those reasons.
It's just a skin on top of the same old stuff everyone else uses, but with more control so you can't reach underneath to see much of the foundation.
That said, it's a fairly deep skin by this point, and getting deeper, but compared to the whole depth of the stack we're talking about, it's still just the skin.
While I don't know enough to comment authoritatively on it, I think with respect to this Apple might have taken a step backwards (at least with respect to percentage of system that is unique, if not in total amount of libraries and code) when it switched to OS X. OS 9 and its prior versions were an entirely unique and custom OS designed at Apple. The whole switch to OS X was taking a UNIX core for stability and building on top of it.
Apple systems are quite different than the norm in many respects, but the scope of this discussion is so large that there's really little they do that's different than everyone else from that perspective, because nobody does anything different. It's hard to start from scratch and ignore approximately 80 years of concerted computer science effort and come up with something useful and usable in any sort of time frame that makes it worthwhile.
I’m a software engineer, I’m perfectly well aware iOS and MacOS run on top of a Unix kernel. But unless I go looking for it, they will never shove this in my face and make it my problem.
Meanwhile, I’m still dealing with Linux-loving colleagues who cannot get their sound or webcam to work reliably and consider this an acceptable state of affairs.
That's largely the fault of manufacturers who refuse to properly document their devices and release functional drivers. I don't think anyone considers that an acceptable state of affairs, merely tolerable if it allows using an open software stack that you can freely modify.
This isn't so much of an issue anymore in my experience. Older webcams and sound cards that work flawlessly with Linux often don't work with newer Windows versions, though.