Trying to chase that elusive "one OS to rule them all" has been less than a roaring success on Windows. They have the monopoly power to shove their inferior crap down everyone's throat, but that's another issue.
It's tempting to see a mobile device (which primarily or exclusively uses a touchscreen for input) and a traditional "desktop" PC (meaning any device that primarily or exclusively uses a mouse and keyboard for input, including regular laptops) as being nearly the same thing, but it hasn't worked in practice.
The Windows UIs since 8 have tried to split the difference, and they have failed. Both of them are a weird admixture of a regular Windows Win32 UI (often using the same dialogs that have been in use for many years) and a touch UI. Some things require the Win32 UI, which does not lend itself to touch use, and others require the mobile UI, which does not lend itself well to mouse and keyboard use. MS has been gradually moving things from the desktop-oriented Control Panel to the mobile-oriented Settings for years, but there are still a number of things for which the Win32 version is necessary, like the Device Manager.
The two form factors have different expectations about what a user can do, though the OS and UI "designers" have increasingly fouled this up with their attempts at convergence.
On my Dell XPS 13 laptop running OpenSUSE Tumbleweed that I am using to write this, I have Firefox set up with a bunch of small icons for various addons. The icons are on the small side if I was going to try to use them with a touch interface even on my 13.4 inch display, let alone the smaller screens on most tablets and all phones. They're easy to hit with a mouse pointer, though, since the point and click events are distinct, and I can see exactly where the pointer is pointing before I trigger the click. The OS/DE (KDE Plasma) helpfully highlights the element when I am pointing at it so I can quickly tell when I am on target. It makes hitting small buttons very fast and easy.
On a touchscreen, you lose sight of the element just before the tap event because now your finger's in the way, and it is a one-stage shot, with no feedback that you're on the right element before the tap is initated. This means that the UI elements have to be much larger, which affects the entire way the UI is designed, not to mention the various applications within.
On a mobile UI, the writer of a given app (or the OS itself) cannot assume the user has a two-stage pointing device that has hover effects to convey useful information or to let that user know which UI element will receive the click event. Most will assume the opposite, since the touchscreen is primary on these devices, and so the oversize UI elements without hover effects will be the norm. Simply adding a mouse pointer (and an input device to drive it) does not turn the UI into a credible mouse-based setup.
The "use the touch UI and just add a mouse pointer" is more or less what Microsoft did with the bits of Windows 8.x and 10 that are mobile oriented... you still have the big UI elements that inefficiently use screen space and don't use the hover effects to their full advantage. When a window is maximized, you still will often have large amounts of wasted (white) space because that UI was designed around a mobile device that does not have a large screen. There's often quite a bit more drilling down to find the option you want, and more UI elements hidden behind things like the infamous hamburger button.
The hamburger button is a UI disaster. It's also nearly ubiquitous, as it has been one of the primary adaptations to the limitations imposed by a touchscreen handheld device. There's a hamburger on the web page I am using right now!
What options are available to me within that menu? There's no way to know at a glance; I won't find out until I click on it. With a web site designed for a real computer, a small menubar could give me the categories of the various options, giving me good information scent about what options or functions are available to me as a user, allowing me to go directly to any one of them if that's what I need to do.
But that hamburger tells me nothing.
I've got plenty of space for a more useful UI, but this site (like so many others) has chosen to make the crippled mobile version the standard for everyone, so all that screen space is wasted. I get all of the downside of the mobile UI even though I am not using a mobile device with so many limitations.
At least it is just a web site and not an entire OS full of applications that make that same compromise for me.
Tim Cook was asked years ago about convergence between Macs and iDevices, and he (to his credit) noted that doing that would necessitate compromises that would result in an inferior experience on one or the other, or both, and that the goal was to make the best Mac available and the best iPad available and leave it at that.
After having seen what a UI disaster Windows has become since Microsoft has tried to chase that dragon, I had to admit Cook had a point, and I am not an Apple fan. I'm used to Apple being the source of the /facepalm moments, like "you're holding it wrong," bendgate, glued-in batteries, the butterfly keyboard, "bravely" getting rid of headphone ports, and so on, but this was one point where they got it right.