"From hardware standpoint, almost any touchpad is capable of producing (relatively) fine-grained input to control the scrolling. Unless touchpad emulates scrolling events in firmware (which some touchpads might rarely do), scrolling precision is largely dependent on the touchpad driver."
[Representing touch events as mouse clicks, as Linux previously did until Xinput 2.1 released in 2012] was great from compatibility standpoint, but for high-precision scrolling it was a showstopper.
The only way to overcome the limitation was to introduce a new API that supports more precise scrolling events. This API happened to be XInput 2.1 Xorg extension which introduced high-precision scrolling events. The API is available since Xorg 2.12, and since version 1.7 the touchpad driver uses the new API. By the way, another thing that was introduced in XInput 2 is separation between device coordinates and screen coordinates (i.e. it’s possible to acquire coordinates with subpixel precision).
"... unlike XInput 2, libinput can report scrolling directly in pixels"
Thus, unlike in Windows, the main obstacle for high-precision scrolling in Linux is not touchpad drivers but applications themselves. Because touchpad input handling is unified, applications are mostly open source and their update is automated, the future of precision scrolling looks more bright on Linux than on Windows, especially for “legacy” touchpads (yet, there’s no X server API for pixel-precise scrolling so far).
[After listing numerous struggles to smooth processsing on Windows/Linux] Now you might ask “…but what about Mac, do they have superior hardware?” While Mac hardware is indeed very good, the underlying reason is different: because Mac OS applies system-wide scrolling acceleration, it can (partially) overcome the problem of insufficient temporal resolution on slow finger motion (as Microsoft stated “If the finger is not moving or is moving very slowly, the frequency can be lower than 30 Hz.”) — by generating small enough deltas to make up for the low event frequency.
Windows supports pixel-precise scrolling only in the combination of Precision Touchpad driver and Direct Manipulation API (yet Direct Manipulation is too all-encompassing and is never meant to used just for input). In Linux, Wayland can generate pixel-precise deltas, while X server still lacks that kind of an API. [Does this actually mean libinput (= "Wayland") can generate pixel-precise deltas, while Synaptics (= "X server") can't? -ed]