I think it's a mix of several things, many have already been mentioned.
Most of the times it's just ignorant people that don't know any better, or choose not to care because it's just a fish. You would be surprised how many people excuse themselves by saying "it's just a fish". Most of us have seen kids at Petsmart asking mommy for an Oscar to put in their 5 gal aquarium. Mommy is tired after working all day and just went there to buy food for the dog, so she just buys the damn fish to get it over with, it's just a fish after all. Sadly it's not their fault, the average person doesn't see fish as smart, sentient creatures that require special care just like any other pet; they see fish as decor. That's just the way it is. You don't have the space or money for any other pet? Get a fish.
The other day I was watching TV and this commercial came on, I don't remember exactly what it was about but I remember thinking WTF... the thing clearly shows a living room and focuses on a fish bowl containing some kind of African Cichlid (Aulonocara genus I think) that was at least 5 inches. So there you have the media telling you this is all you need to keep African cichlids:
I have since then seen the commercial several times, and I only watch TV for about half an hour before I go to bed, maybe every other day.
Not only that but I've seen several shows over the years were fish are made fun of, or used to ridicule people. The general public sees fish as the cheap, disposable alternative because they've always been told it's OK. And I know this because that's the way I saw it before I started keeping them.
On the other hand there's fish keepers who know they are doing it wrong but are just deluded and refuse to admit so. AquaPishFimp comes to mind. "If you feed your Oscar exclusively on broccoli it will only grow to 2 inches and be perfectly happy in a 5 gal tank, just like thousands of years ago when people only ate fruits and grew to 4 feet". lmao. He has a degree on Ichthyology sponsored by Walmart and Wikipedia.
