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.

[1] https://en.wikipedia.org/wiki/Fisheries_subsidy [2] http://archimer.ifremer.fr/doc/00222/33368/35784.pdf