As someone who builds custom rigs on a commercial basis, what i can say is that i will rather sell complete systems then sell mountains of GPUs
GPUs have shitty margins compared to other PC components (especially for things like chassis, accessories etc).
If i sell all the stock of GPUs i have onhand to miners, i not only make less in terms of profit, i generate bad karma with my primary customer base which are gamers and also all the items which i actually make more decent margins on end up sitting on the shelves collecting dust instead (and which i already paid for)