I wonder that too. Rats are smaller than chinchillas, much smaller but most people buy chinchillas cages because there is nothing else on the market.
Adult male rats can go into chinchillas or FN cages but I would never use such a cage for a female rat. They are so much smaller than boys.
Hamster cages are way to small for rats, even a small female rat would not have enough space in it.
Rats are sold in most pet shop as they are becoming a very popular little animal, but why on Earth are they unable to sell the proper habitat for them???