One is reminded of the saying, "once a metric becomes a target, it ceases to be a good metric". Number of papers published in (almost any) journal, has long since come to be viewed as a target that you have to hit in order to get tenure, grant funding, etc.

Another way to say it is, if there is a problem with bribery, what is it that was supposed to prevent that? Currently, the answer is approximately "nothing". There is nothing in the current scientific publishing system that is intended to prevent bribes, you just aren't supposed to do that, or accept them if they are offered, but there is no mechanism to suppress it. Therefore, it happens.