I must of misunderstood your post. I assumed you meant that they recycled all the memory chips from old GDDR5 GPUs and ended up using those in some newer GPUs as rebranded chinese ones. What you are saying makes perfect sense.
Basically if only 1 memory chip is faulty and you can still sell the GPU for >$100 then its worth the labor to just replace that 1 memory chip. Probably will take less than an hour from start to finish. I had an old old Bitcoin SHA256 ASIC called the Jupiter (or Saturn) ASIC. This was back in 2014-2015 and I had many of the VRMs die on some units and I basically removed the dead ones and took working ones from other boards. Since I was only replacing 1 VRM instead of all of them it was worth while.
I do think they can still find profit even from reballing the whole card instead of just 1 or 2 faulty memory. It's because of perspective where we think they do it from a small kind of shop with little tooling. But what about at the industry level with a lot of BGA machines and scale-up processes? I think they can do it much faster, with high precision as well. Once old junks rebrand as new, it may be worth it after all since they got the memory and GPUs for dirt cheap price. All you need is the same pcb with the same memory layout -> all hard work can easily turn into automated.