Even when you get those stolen GPUs placed on your porch for free, you still have to set them up in mining rigs, install software, cables, cooling et cetera, and you need to cater to the hardware once it is running and fix problems coming up.
The difference between Bitcoin ASICs and GPUs in performance is large enough to make the expenses with these activities not worth the hassle, even if you get the hardware AND the power for free.
