Here we get 5.1 speakers, that is a woofer, with power supply, FM radio, SD/USB, built in amp, 4 satellite speakers, for $40 and up retail.
The factory gets maybe $25, if that.
So you have slightly better cabinets, and four drivers , or two drivers and two cone tweeters.
Doable, if tell the supplier you need 50,000 sets, for about $50 in China.
Add shipping, and remember Amazon delivers direct, they do not have expensive retail shops with the hassle of real estate, people roaming around and so on. That is a big saving, no display, no parking, no sales assistants.
Quality? Nobody seems to care anymore, or are so used to the sound of tiny speakers and earphones that if they listen to a good system, they will not appreciate it.
All show, little substance.
As mentioned above, somebody looked at the sales, and decided it was profitable to trade under their own name, that is all.
There is a software called SAP, which is used by supermarket chains here (among others), to track sales, and stock replenishment levels across locations. Fast selling items are easier to spot, and the analysts informed the management, and it went from there.