The problem with these types of things is that the people who are qualified to do good reviews are also the sort of people who can typically get a far more interesting job building stuff, rather than just reviewing code. It's work that does require a certain level of skill, but at the same time is also quite boring.
And that more interesting job will probably pay better as well.
Id rather hire a senior dev as a reviewer and a mid dev as the coder at a company. pay the reviewer more since they will be dealing with shit practices and having to train the dev.
Not only that, but properly reviewing code would take forever. Heck I don't know how many senior engineers at my fancy tech company could do it and reliably spot problems.
And that more interesting job will probably pay better as well.