The fish you buy even from reputable suppliers was very likely flash frozen on the boat before it ever reached land. Many fishing boats are out for several days or weeks at a time. So just because it's only been on land a short time, that doesn't mean it's recently caught.

This is a good thing because many fish harbor parasites that are killed when flash frozen. Your home freezer doesn't reliably get cold enough to take them out. You can reliably kill them by cooking very thoroughly, but since fish is such a delicate meat, the line between "may still have wriggling worms" and "overcooked, dry, and unpalatable" can be quite fine.