Because of the industrialized pet industry, the likes of PetSmart. They've convinced people that small pets like fish are products that can be thrown away instead of living individuals. They'd rather your small pet die in two weeks because that just means you'll be back to buy another one, so they've launched a propaganda campaign against any sentiment of responsibly caring for them and fooled the majority of people into thinking that it's just their normal lifespan and they need way less care, equipment and commitment than they actually do.