Becuase they're cheap.  From Wikipedia:

   Subversion currently lacks proper repository administration and
management tools. That is,
   while it is very capable when data is added to the repository, it is
much less capable at managing
   the repository as a whole. For instance, it is sometimes necessary to
make permanent edits to the
   repository to change the structure in which versions are held or to
permanently remove data that
   was checked into the repository in error. Subversion does not have tools
which allow this to be done.
   The check-in level tools allow files and directories to be moved or
deleted but earlier revisions will
   always hold the data in the old structure or hold the file that was
deleted. The current solution to this
   sort of problem involves 'dumping' the repository, editing the resulting
(possibly large) text file, and
   then recreating the repository. For simple renaming or removal of files
this is fairly straight-forward,
   but other alterations can be more complex and hence error-prone.

Not to mention that a closed source tool easily has open source
equivalents. A closed source game generally won't have open source
equivalents. An open source railroad game may exist but it won't be
entirely a substitute for Railroad Tycoon, for example. An open source
first-person shooter isn't going to be an exact replacement for Quake
4, and so forth.

Games are one area where closed source can still admittedly compete.