Because in many cases, it is. Unfortunately, there isn't as much communication between the GUI people and the kernel people in the Linux community as there is between those same groups at Apple Inc. Not to mention, there are multiple competing groups of GUI people in the Linux community making coordination across these levels difficult. Also, there are many competing interests working on the kernel who might oppose kernel-level optimizations which favor desktop usage at the cost of, for example, server usage. As a result of these and many other factors, Linux's desktop UX remains far less optimized when compared to macOS's desktop UX.
As with most of Linux's rough edges, however, this is trivially fixable if you're technical enough. Of course, that's exactly macOS's advantage. If you want the best experience on macOS, you don't need to be technical.
Personally, I run Linux with a tiling window manager and lots of custom keybindings and little scripts. Having used the default macOS experience a few times at work (on machines far more powerful than my dinky laptop), I can assure you that my highly customized setup feels far more responsive. On the flip side, it required a lot of up-front investment to get it to this point.
I never felt the UI responsive difference between Linux and MacOS but Windows (including freshly installed on powerful many-core machines when) is a different story. The number one reason I ever switched from Windows to Linux is the latter always feeling way more swift - UI responsiveness always remaining perfect and some background tasks also working much faster. And I never actually used lightweight WMs - only KDE, GNOME and XFCE. The first time I've noticed some slowness in Lunux was on RaspberryPI (4, with the default LXDE).
I think the main advantage with macOS is that it's above all else a system designed to be used interactively, as opposed to a server, so they don't have to put out a configuration "good enough for most things".
I also run Linux with a tiling window manager on an old machine (3rd gen i7), and it flies. One thing that made a huge difference in perceived latency for me was switching the generic kernel with one having Con Kolivas' patch set [0].
I'm using Arch and sign my own EFI binaries, so I don't care about out of tree patches, but for Ubuntu users and similar who don't want to mess with this, there's an official `linux-lowlatency` package which helps [1].
As with most of Linux's rough edges, however, this is trivially fixable if you're technical enough. Of course, that's exactly macOS's advantage. If you want the best experience on macOS, you don't need to be technical.
Personally, I run Linux with a tiling window manager and lots of custom keybindings and little scripts. Having used the default macOS experience a few times at work (on machines far more powerful than my dinky laptop), I can assure you that my highly customized setup feels far more responsive. On the flip side, it required a lot of up-front investment to get it to this point.