Because you are incorrect. Free also can be affected. It boils down to the OEM graphics driver support for OpenGL, OpenCL and WebGL.
I have an old machine with Intel integrated graphics that is “out of active support” and for about 4 months last year it would not load my.sketchup / SketchUp Free. The SketchUp Team had to fix SUF so it would have “fall back” mode for older version of WebGL support.
The bottom line is that browsers are also applications that can be affected by graphics driver bugs or poor standards support. But because of the importance of browsers, they are quickly patched. (Most users of Chrome are unaware of all the automatic updates that happen, as they are done transparently.)
Because SketchUp v2018 is the current version in maintenance mode.
Since they switched to a yearly release cycle for the SketchUp 2013 version, there is only time enough for the initial release and one or two maintenance releases within say 3 months.
After this, they must begin work on the next major release as they have perhaps 6 months before code freeze and the testing phase must begin in order to make a November release.
They are a small software group and just do not have the people and resources like Microsoft does, to maintain and patch applications out to 8 years.
So after about February, each year, you’ll likely not see any more “patches” released for even the current version.
From the EULA, section 7. Warranty Disclaimers, …
TRIMBLE MAKES NO WARRANTY …
(i) THAT THE SOFTWARE WILL MEET YOUR REQUIREMENTS,
(ii) THAT THE SOFTWARE WILL BE ERROR-FREE OR BUG-FREE,
(iii) REGARDING THE SECURITY, RELIABILITY, TIMELINESS, OR PERFORMANCE OF THE SOFTWARE,
(iv) THAT ANY ERRORS IN THE SOFTWARE WILL BE CORRECTED,
Legal mumbo-jumbo thought up by lawyers, but it does seem a bit oxymoronic when the marketeers are saying the opposite kinds of things about the product.
Do not confuse the 3 year technical help “support” with the yearly software “maintenance”. They are two different “facets” of the product life cycle.