I couldn't disagree more! What happens on a PC after POST? You see logo/loading screen of the disto. Next comes the login screen: At least 3 different programs are used for that in the common distros and they all behave a little different and look different. You type in your password, you enter your desktop environment. GNOME, KDE, Unity, MATE, Cinnamon, Pantheon, Xfce, Lxde... shall I continue? They all look completely different, behave completely different, come with different standard apps. Speaking of apps - DPKG, RPM, Pacman, YaST - apt, synaptic, dnf, yum. Shall i continue? NOTHING is the same! How much do you actually "see" from the kernel?
Now have a look at Mac OS X or Windows: Every iteration of Mac OS X from 2001 to 2018 is entifiably Mac OS X. Menubar (black font on white/gray background), Dock on the bottom, never changing Apple logo in the upper left, traffic lights on the upper left of every window. Same applies to Windows. Yes, every UI element got bigger, transparency was added, stupid decisions where made to make the taskbar look more like the OS X dock, but in the end it still Windows, because it always workedthe same.
And now turn back to Linux. Take Ubuntu for example. 8.10 was solid, stable and ugly. In short: "Linux". 9.10 looked better, 10.04 changed the look completely, 10.10 changed functionality, then GNOME went and Unity came, banshee and rhytmbox came and went, Amazon dock icon came, the dash was rebuild and looked even worse but worked better, Unity and Mir went and GNOME came back, then wayland came, standard apps changed every now and then. Menubar in the window, global menu bar, hidden global menu bar, hamburger button. Add Software, Ubuntu Store, Gnome Software. Ubuntu, Kubuntu, Xubuntu, Lubuntu, Gobuntu, MythBuntu, Ubuntu Mate. And that was just Ubuntu! That was only the very same distro in many iterations!
Now do tell me: What has NOT changed in the eye of a not technically adept user?