Why do stores keep Betta fish in small containers?

Betas are fighting fish and can NOT be kept together, they will kill each other. Betas won't get much bigger than those containers in the time it takes to sell them, unlike the goldfish who grow quickly. Betas are lovely, come in array of species, and colours. They breathe air and don't need a filter. A glass container with the scalloped top in the floral department is a fine size for them.