That's just, like, your opinion, man. There are other bits of infra that integrate with it quite nicely, and would integrate with something like Git quite poorly. One of those things is their code review system. The closest thing I could find to it outside Google is Gerrit, but it's a tremendous pain to set up and use, and it's but a pale shadow of Google's internal tool (Critique).
And also, one does not preclude another: Google has a git wrapper on top of Piper, so you can spend your entire Google career not even touching Piper directly if that's what you prefer. And Piper went beyond the "Perforce model" in ways I can't disclose here.
I've used lots of review tools and worked a bit on Google's review tool and on ReviewBoard in the past, and Reviewable is better than all of them in my opinion (or at least, better than when I last used the others).