Because it was not added until SketchUp 2014.
If you are using v8 Free, it would be better to also install SketchUp Make 2014.
- SketchUp Make 2014 Download Links (provided here)
It uses Ruby 2.0, and fixes many bugs that v8 and v2013 have.
Also it is difficult to use the Extension Warehouse and 3D Warehouse from v8.
In your forum profile, the SketchUp bitness will be 32-bit. (The 64-bit builds of SketchUp did not start until v2015, which do not run on XP.)
You’ll want to go here and read the one for the languages you already know:
Ruby From Other [Programming] Languages
Unfortunately, SketchUp v2013 and earlier, use Ruby 1.8, which has many bugs and limitations. On PC it has problems with Unicode string support.
Ruby 2.x does not have these issues. Also it is faster than v 1.8.
Also SketchUp 2014 and later have come with the full Ruby Standard library. For earlier versions (7 thru 2013) you must install it separately: