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.
You see this in aviation and heavy rail transport as well for a similar reason. The critical items like the engines, control systems and emergency equipment being expensive is obvious, they’ve been heavily tested and made super resilient with very strict certification criteria, but it even extends down to things that has pretty much zero impact on safety. That flimsy seat back table that can barely hold your laptop and that hard as a rock cushion cost the airline far more than you can imagine, costs which are inevetably passed onto you.