Hand-held touchscreen devices require big UI elements. Human fingers span tens of pixels on a 5 inch screen, and there's no feedback indicating what element will receive the tap before the tap takes place (in contrast with a mouse, where one can clearly see the position of the pointer before clicking). If one-handed operation is desired, it's even worse, as the thumb spans more pixels than the other digits. Additionally, one-handed operation favors the "portrait" orientation, which is seldom used with traditional PCs.
A system using the large controls that work with small-screened touch devices must use screen space sparingly, as there's not much to work with. Onscreen buttons and top-level menu options are sparse, with many actions requiring quite a bit of drilling before you can find what you're looking for. It's a necessary annoyance when one is dealing with a small screen intended to be used with human fingers.
A traditional PC interface has none of those limitations. A mouse pointer can easily and reliably hit a target that is only a few pixels wide, and the display is large enough to allow many more onscreen UI elements than would be feasible with a mobile device. To push that same touch-oriented UI onto a regular PC would be to waste much of that screen space, and to impose the annoyance of having to drill down through layers of menus when there is plenty of screen space for a faster, more elegant solution. It's a necessary annoyance with a mobile device, but an unnecessary one on a PC.
That's one of many areas where Windows 10 fails. Many of the menus and dialogs built into the OS are of the touch variety, with no standard versions available. One example is the touch-oriented Settings app, which is slated to replace the mouse-oriented Control Panel that has worked for us for decades. Changes like that are a step backwards in usability and UI design for the PC user. We're being asked to accept the compromises to make Windows work on mobile touch devices even though we aren't using them. It's all cost and no benefit... well, there is benefit, but it's not for us.
Sorry, Microsoft, no sale on that one.
The code for traditional Windows menus already exists from previous Windows versions, and it could very easily be selectively used on non-touch devices with no noticeable increase in the installed Windows footprint. In fact, I would guess the old Win32 dialogs from previous versions of Windows are still there in Windows 10, but are simply turned off, which would be normal for Microsoft.
If you use a resource editor to examine the hundreds of files that make up a Windows installation, you can see dozens of obsolete Windows dialogs from Windows versions long forgotten. These dialogs take up so little room (on disk or in memory) that it's better to leave them in than to take the slight risk that removing them will cause unexpected regressions in stability. Clearly, they're not taking up much room, or MS would undoubtedly have removed them from the code base years ago if there was any tangible benefit to doing so.
So, if that's the case (and it is), what's the rationale behind forcing PC users to muddle through with touch-oriented dialogs? Are you there, MS?