Note that Windows is a similar mess. From the very beginning, you had plenty of different GUI libraries - MFC, WTL, WinForms, Delphi, Xwt, wxWindows etc. (let's not even mention Java, the perpetual red-headed step child)
Every office version introduce some new "default style" that everybody had to emulate (flat toolbars, ribbons...), resulting in a plethore of subtly different applications. Does your menu have icons? Where is the preferences dialog?
And the only salvation for the driver mess is that apart from Nvidia, Intel and ATI, every other manufacturer has left the picture. No more Tseng, S3, Matrox etc.
But Windows is immmensely popular. So you have to go through those problems. Linux isn't that hard, but it still might not be worth the effort. OS X does pretty well in that regard, not that popular, either, but very homogenous. At least since the days of RealBasic apps is finally over ;)
For games you don't care about the window system, what's critical is a 3D API, input and sound. MS got this right early with D3D, DirectInput and DirectSound. And MS forced hardware vendors to commit to these APIs which was a good thing.
The problem with Linux (above the kernel) is that everyone is pulling into a different direction.
Of couse games do not care about UI toolkits, but the excuse against linux was always Gnome-KDE or Gtk-Qt-Motif-Athena-whatever, even through games do not care about these things (see also your grandparent post).
In Linux, there is glx[1]+OpenGL and SDL. The problem (quality of OpenGL implementations) is not unique to Linux.
Yes I remember now that the Valve SteamOS devs recommended to use SDL2 instead of going directly on the OS APIs, since it takes care about a lot of small problems and differences. So basically SDL would take the role of DirectX.
And the only salvation for the driver mess is that apart from Nvidia, Intel and ATI, every other manufacturer has left the picture. No more Tseng, S3, Matrox etc.
But Windows is immmensely popular. So you have to go through those problems. Linux isn't that hard, but it still might not be worth the effort. OS X does pretty well in that regard, not that popular, either, but very homogenous. At least since the days of RealBasic apps is finally over ;)