The things I can think of is like this.... The average pet owner has not had the experience of caring for 100s of bettas. A breeder or someone who owns many many bettas sees illnesses more often and is able to learn signs faster than the average pet owner. Generally a breeder is also more guaranteed to do water changes often enough because bettas not only are their passion, but their business. Losing bettas means losing money, and while they aren't just out for money, it is a big deal. A kid in middle or high school has so much drama and other passions that we can often forget to change water. In a container so small, that just can't happen. The five gallons gives us the extra buffer time before things get bad. If you keep a betta in a small container.. You must have a set schedule and you must follow it right!
Another thing is it's how we wish to care for them. Fish do love to play in plants. Doesn't mean they are necessary. The plants also look nice AND keep the water clean, making it easier on us. :) You could also keep a betta in a tub. It gives them privacy and is cheap and easy to have multiples of. But then you can't see them. Which to be honest, is at least partially why we get them :)
Same is true in the reptile world to be honest. I know I make this comparison a lot, but it's true.
These are gecko breeder racks. Every single leopard gecko is properly housed, given the proper heat, enough space, water, food, and they are all healthy and happy. It is bare minimum. They don't have hides, decor, plants, and they really could use more space if taken as pets.
Here's a pet owner's leopard gecko set up. The gecko is properly housed..given great heat and humidity...given a hide since there is so much space, and there is a ton of room to explore.
That same gecko in the tank could do just as well in the breeder set up. But one issue is the 'out of sight out of mind'... A pet that we can't see is often a pet we forget. If you aren't as dedicated as many breeders are, mistakes happen, you forget to feed, your animal suffers. A tiny cage could mean one hot day KILLS your gecko, as they are trapped. In a big cage, if your heat pad overheats, the gecko can run to the cooler end of the cage, so if you don't catch the error, your gecko still lives. Just the whole room for error thing.
Most of the reasons we say they need big tanks is because they need heat and most heating for small tanks is unreliable. If you provide heat, the size isn't as important. YES, they love to swim. I'd never put a plakat in a 1g, because I know how much mine loves to zip around. I have seen perfectly happy halfmoons in 1gs though!
I think a pet owner could keep a betta in a more simple setup. If they do good water changes, provide heat, and make sure the betta is healthy..than go for it. If you have enough experience with bettas you can see "this one is stressed", so you can give it a place to hide. You can see a fish who needs more exercise. That could mean flaring more often, or giving them a larger tank to swim in. If you don't know your fish as well, you might mistake a sick betta for a chilled out one. In a small tank, that will mean a much more likely death.
Big tanks=easier for few fish
Small tanks=easier for many many fish!
But it's up to you I think. you are the only one who knows your animals. If you see that something isn't working, change something.
If you want to keep your fish in 1g containers but they simply aren't happy in them.. Put them in a larger tank. If they are fine, then feel free. You know your fish, and you know when they are happy and healthy.
I see clean, warm water in these barracks. It is simple, yes. But I don't think the fish are unhealthy.
And I see a warm, clean tank here with a healthy betta.
I think the health of the fish is what counts the most!