Once an animal is dead, the enzymes in its own cells start degrading its tissue. Enzymatic reactions are complicated, and only work well within a narrow temperature range.
Land animals have a body temperature similar to humans. When they are slaughtered and the flesh is chilled, the enzymes slow down their work a lot.
Fish are "cold blooded" animals. This is usually a misnomer - a snake lounging on a sunlit rock can get quite warm. But fish live in water, which tends to be cold. Their enzymes are very active in the temperature range found in refrigerators. So non frozen fish starts to spoil, and this can't be halted.
People generally use spoilage as an indicator for unsafe food, and are probably not aware that it doesn't apply in this specific case. Also, the decomposition of fish produces some amines which are usually perceived as a very objectionable smell. So people just prefer eating their fish before this decomposition process has progressed much.
Beef's decomposition is slowed down a lot in a refrigerator, and it takes different chemical pathways which do not produce a fishy smell. People even like some of the smells which appear during early decomposition (the gaminess of aged beef). Also, specifically for beef, aging provides tenderization. This is not needed for fish, whose muscles fall apart easily without aging.
