Reasons behind why average fish only live for 3 weeks after sold

Well, as far as I remember, they would usually sell small fish bowls for those who don't have one. Indeed, small children won't usually have the knowledge of how to look after a pet properly, and parents would often just provide fish food. But it's often kids' first introduction to owning a fish. So, even though it sucks that goldfish are treated badly, I'm sure some of these kids ended up as experienced fish owners.