The short answer is that when Google started, Perforce was the best kid on the block. Once you get as far down the road as Google, it's hard to change that incumbent, even if culture dictates the use of open-source software.

The other problem is that Google just has a whole lot of code. They've got engineers cranking out code all day all over the world. Working at that sort of scale rules out other alternatives (note that Google hired the Subversion guys, and Google isn't using Subversion... this should tell you something).