Maybe I'm just dreaming.
To sell Microsoft Surface, among other things.
Windows Phones are gone, but people still use Office on touch devices: MS Surface tablet, and other manufacture's Windows tablets or hybrid laptop/tablet devices, or just touch-screen laptops¹ which are a thing, and increasingly the web versions of office that might be used on anything modern including iPads.
> I don't understand why they removed the lines between buttons.
The usual reason for things like this is that it makes things look cleaner, or more modern⁴, which it does. Though IMO it has a detrimental effect on discoverability (it that a button, a link, or just some text?!) and navigation, even for the modern touch-first idea it is sometimes aiming at (where are the edges of what I can touch?).
My biggest irritation with modern UIs is focus indication on the desktop. It has been a problem for many years but only gets worse as time goes on: it can be difficult to see at a glance, especially over multiple screens, which window currently has input focus because many app families use different window decorations and even within one app the active/not distinction can come down to the difference between one shade of off-white/black and another shade of off-white/black (MS Office and Firefox are both guilty of this).
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[1] I know people really like touching their screen where possible instead of using a mouse or trackpad, I sometimes find it preferable myself² though not for Office & similar tasks.
[2] and for the most part I'm an old-fashioned keyboard-or-die person, regularly bemoaning bad design decisions³ (or simple laziness of not implementing standard shortcuts) that force me to leave the keyboard to mouse/trackpad/touch
[3] which can't be good for accessibility reasons, as well as not-irritating-the-like-of-me reasons
[4] though given how quickly fashion changes, I'm of the opinion that “to look modern” should never be a design consideration in a product intended to last into next year!
That's smart! : )
And if using a touchscreen for that is a pain, then at least optimize for a mouse instead of something noone will actually use.
With computer mouse you can use much smaller and denser controls of course.