One reason is that many governments are essentially paying people to take fish out of the ocean [1], to the tune of tens of billions a year. Japan, China, and Taiwan are some of the main players. They do it to keep fishermen employed.
Related to this: because technological change has made it really cheap and easy to catch fish [2], and up to now, that has more than offset the decreased productivity from damaging our fisheries.
But then why is fish still so cheap and plenty of it avaiable, which was the main point?
(but again, I don't know that, I am just refering to the argument)