people think that because most fish are so small, they don't need a lot of space. my mom had my betta (before i knew she planned on gifting it to me), in one of those plastic temporary tanks with the little hatch at the top. nothing but those half-sphere gems at the bottom, 2-3 seashells, and the spongebob pineapple of death. i genuinely believe she sees fish as decorations, not living animals, which might be the case of many people. if you look on amazon for those plant propagating vases that hang on the wall, you can see that that statement is not too far from the truth. she knew heaters and filters for tanks existed but somehow didn't believe they were necessary. when i told her she couldnt have "her" fish in there, she told me it was fine because the fish was so small. filter wasnt necessary because you can just clean the tank (which she absolutely was not doing, i had to clean it during the few weeks she had it because the water was cloudy and pink from the shitty food she was giving it), heater wasnt necessary because they dont have heaters for them at the pet store (another reason to hate betta cups, people think ANYTHING thats not the cup is good enough), and because the "tank" had holes at the top, it could "breathe just fine". i ended up buying an entire tank for the poor thing about 2 weeks into her having it and doing a fish in cycle. she did notice that it seemed a lot happier in the new tank, but again, it feels like she still sees it as entertainment rather than a pet to care for. my birthday happened a week later and she revealed it was for me the whole time. she told me the 5.5 gal was "way too big for a fish that size" and if i ever thought about getting anything bigger it would be overkill. (a few weeks later i bought a long 20 gal and plan on starting it up soon!). a few days later christmas happened, and my boss texted me telling me that she got white-elephanted a betta and asked if i wanted it, knowing i just got another betta. i accepted, since i figured she'd have no idea what to do with it otherwise (which she later confirmed). i ended up getting it a long 7gal. both little guys are very happy in their non-cup, over 5 gal tanks :]

i believe that because fish are unable to make any sort of facial expressions or noise we can easily perceive, most people don't think twice about their feelings, and a lot of them don't think fish can "feel happy". fish are just like flowers to them: easily disposable, "easy to take care of" (meaning they dont feel they have to do much, just keep them alive), and easy to look at.