For one thing, it is so very customisable that people have their own workflows that they love and they won't even consider anything else.
For another, there is 51 years of UNIX™ cultural baggage. For example in the simple matter of text editors. There are two big old text editors in the UNIX world, both dating from the 1970s. Both are incredibly powerful and capable, but both date from an era before PCs, before screens could display colours or formatting or move blocks of characters around "live" in real time, before keyboards had cursor keys or keys for insert, delete, home, end, and so on.
So both are horrible. They are abominations from ancient times, with their own weird names for everyday stuff like "files" and "windows" -- because they are so old they predate words like "files" and "windows"! They don't use the normal keyboard keys and they have their own weird names for keyboard keys, names from entire companies that went broke and disappeared 30 or 40 years ago.
But people still use these horrible old lumps of legacy cruft. People who were not yet born when these things were already obsolete will fight over them and argue that they are the best editors ever written.
Both GNOME and KDE are very customisable. Unfortunately, you have to customise them in the ways that their authors thought of and permitted.
KDE has a million options to twiddle, but I happen to like to work in ways that the KDE people never thought of, so I don't get on with it. (For example, on a widescreen monitor, I put my taskbar vertically on the left side. This does not work well with KDE, or with MATE, or with Cinnamon, or most other desktops, because they never thought of it or tried it, even though it's been a standard feature of Windows since 1995.)
So if you want to customise GNOME, you'd better hope that someone somewhere has programmed the customisation you want, and that their extension still works, because there's a new version of GNOME every 6 months and it usually breaks everything. If you have a broken extension, your entire desktop might crash and not let you log in, or log out, or do anything. This is considered perfectly normal in GNOME-land.
Despite this, these two desktops are the most popular ones around. Go figure.
There was one that was a ripoff of Mac OS X, and I really liked it. It was discontinued a few years ago. Go figure.
Rather than ripping off other desktops, the trend these days is to remove most of the functions, and a lot of people like super-minimal setups with what are called "tiling window managers". These basically try to turn your fancy true-colour hardware-3D-accelerated high-definition flat-panel monitor into a really big glass text terminal from 1972. Go figure.
There used to be ripoffs of other OSes, including from dead companies who definitely won't sue. There were pretty good ripoffs of AmigaOS, classic MacOS, Windows XP, Acorn RISC OS, SGI Irix, NeXTstep, Sun OpenLook, The Open Group's CDE and others. Most are either long dead, or almost completely ignored.
Instead today, 7 out of the 8 leading Linux desktops are just ripoffs of Windows 95, of varying quality. Go figure.