They aren't. If you ever had a convertible laptop, it would enbiggen the UI in tablet mode and reduce it in laptop mode.

I rarely used tablet mode but genuinely miss having a touchscreen.

Not anymore, a lot of apps (from browsers to the system UI) adapt to whether you have a touch screen and whether you use it. For example, Windows Explorer and Edge (and Chrome) still use (relatively-)compact widgets on touchscreen-less computers.