(1) Good hardware with good software support goes a long way. I'd say that iOS is much more of a polished OS than macOS. No wonder, considering that it's the iOS devices that fuel Apple's profits, not Macbooks.
(2) For a lot of people, an iPad is more of a consumption device, or a device to run 2-3 specialized apps (e.g. for drawing or music). Maybe they would be glad if iOS offered wider universal computing capabilities, but their choice of a powerful and mobile device is pretty limited.
(1) It depends, as ever, on one's needs. iOS is limited by design in order to deliver the things it's intended to do more smoothly. There's a lesson there.
(2) Many people really only use any computer as a consumption device, but the general ding of iOS as "read only" has always rung hollow to me. A large number of people work primarily with text -- emails, posts, documents, spreadsheets, presentations. An iPad with a keyboard is a GREAT tool for them, and that's not a "read only" use case.
As an aside, when I last updated my camera kit I added an iPad Pro to use for photo processing and culling when traveling instead of a laptop. It's actually GREAT for that, too, but this level of power is relatively new to the platform.
Your #1 is not a valid statement. OS, be it Mac, iOS, WatchOS, TVOS has a huge shared codebase. It's all OS, tweaked to the particular need of the hardware.
(2) For a lot of people, an iPad is more of a consumption device, or a device to run 2-3 specialized apps (e.g. for drawing or music). Maybe they would be glad if iOS offered wider universal computing capabilities, but their choice of a powerful and mobile device is pretty limited.