Because the Cubs are just better.
Probably because fish spend their entire lives in a fairly constrained low temperature due to the chemical and bulk thermal proprties of water. Some polar ocean fish, for example, may never encounter water that isn’t within a few degrees of 0C (32F, the freezing point of fresh water). Few aquatic environments reach 20C (68F) without artificial effluents - and those few rarely have any fish. Fish are also cold-blooded, remaining near the temperature of their environment
“Meat” (land mammals) must be able to endure much larger environmental swings - near or below 0F in the winter to 40C ( 104F) or more in the summer. The temperature may rarely reach 40C in a given locale, but a species must survive once-per-century or once-per- millenium heat waves. Mammals are also warmblooded, with an operating temperature significantly above the ambient environment 37C in humans) The structural proteins and enzymes of are designed to survive the extremes they might encounter without getting out of control or denaturing (losing their operational structure) A land animal has billions of years of adaptation to land conditions, a fish has bilions of years of adaptation to aquatic conditions
Most spoilage is due to the action of cellular enzymes and/or bacteria. Enzymes (and most other chemical reactions) follow Arrhenius’ Law, which posits a doubling in reaction rate for every 10C increase in temperature. Living flesh exists through a fairly delicate balance of the actions of thousands of enzymes. Cow meat in a standard US fridge (42F, 7C) is well below the design temp of its enzymes, which may therefore have 1/8th the activity they would have in a living cow. For an arctic ocean fish, that same temperature is above its normal ambient temperature, and the fish will steadily go bad, even if scrupulously cleaned and prepared before storage.
Insect, plants, and other non-“meat” land organisms tend not to spoil nearly as quickly as mollusks, seaweed, and other ‘non-fish’ aquatic organisms, which tends to confirm this hypothesis
Of course, this raises the question of why cooking tends to preserve food. Cooking denatures enxymes and orther proteins, by changing its underlying structure destroying almost all of its intrinsic enzyme action. Cooked foods spoil almost entirely through external bacterial (or fungal) action