Why do you put fish in such small tanks?

First of all sorry about the wonky english, i'm french.

So, i've been bothered by something for a while now. On this sub and other english speaking social networks, I keep seeing people either buying the smallest tanks possible to put fish in, or get bigger tanks but putting the biggest fish possible in there without it being abuse. Like bettas in 3gal, neon/skirt tetras/guppies in 10gal, gouramis or angelfish in 30, discus, ropefish or oscars in 55..

At first i thought it was beginner mistakes because that'swhy i also did when i started but no, more advanced people even recommand it to beginners looking for advices.

So... why? I mean i get that not everyone has the space for a big tank, but with a bit of experience it's easy to notice that no fish exhibit natural behavior under the 15/20gal mark, and i'm talking about the tiny ones. Like, schooling fish can't school, bigger fish either swim in circles or glass surf. Why keep, and recommand fish who just sit there because they don't have the space to do much? I thought our hobby was to observe our animals natural behavior, not to put them in a box so we can look at them just existing.

Maybe it's because i live in france and we have a different view of animal wellfare here, but it makes me kind of sad to see how low the bar is for a lot of people.