Because there's no need when your model is "set up the store, sell as much low end crap as you can in 3 months, shut down store, rinse, repeat." They aren't playing long games, they're moving cheap unbranded surplus quickly. They're using excess and part discards (or parts that fell off a truck) from branded products. China has essentially no protection of patents or ip, so factories follow the product leaders, copying whatever is currently in demand.