Well, that is the problem. No one (well, say very very few) do code reviews on their own -- they do not provide as much benefits as coding (learning and reputation wise). Its a bit like the problem in academic publishing: Replication trials are not prestigious enough, so they are done far to rarely, albeit they are very important.
There are companies out there, that will do it for money, and many great open source products have been reviewed that way. But I don't think this happens often enough.