Why do stores keep Betta fish in small containers?

Most people will not be able to keep up with (daily water change) for long even if they have set such goal at the beginning. Most people will get lazy quickly. It is the very reason a filter is needed to run 24/7. Not everyone has a room temperature of 70F+ all year long. Betta will become less active when the water is too cool for them, and in longer term their immune system gets weakened which makes them easily to catch diseases and die.

Besides, most pet stores do not have the most educated pet owners working as employees. Many of them will tell you that your water is fine when there is ammonia and nitrite. They would think "it is acceptable level" while no ammonia and nitrite is acceptable at all in an aquarium. They should be at 0ppm.

The average lifespan of betta is no more than a few weeks after sold. Most of the buyers do not own a proper sized fish tank, nor a heater and a filter. You can see where the problem is. If they get all of the above which don't cost much at all, the life span of betta would be a lot higher. In the process, they save themselves a lot of trouble too. Only weekly partial water change is required for a well equipped and well maintained aquarium. Betta is happier, you are happier. I don't see why not.