This doesn't happen though. There's nothing stopping you from setting up your own fish whole selling company if you think that is lucrative. Sure there will be regulatory hurdles, but if you can make a case that you can do it for cheaper while still making a profit, you can find an investor to pay for your lawyers.
The fisherman wants to sell all their fish with a price that they know in advanced. However, the packer only wants to the fish that they know that they can sell to supermakets, and doesn't want to buy more than they have to. Without someone assuming the risk of holding a bunch of frozen fish for a long time, the fisherman wouldn't being fishing as much, which means less fish ends up with the customer, and less value being created.