When you learn more about factorization you will discover that for factoring numbers of 100 digits or more the best way to narrow down the options to use trial factoring on is by using a process called GNFS sieving. This process simply cannot be efficiently done on graphics cards. Graphics cards can help with the process (step 1) but the longest part is step 2 so CPU's have had an overall advantage. Ideally GPU would be used in tandem with a CPU... which just so happens to be a GREAT way to block botnets or server farms and give the advantage to personal computers.
Can you
prove an upper bound on parallelization? In other words, can you prove that the advantage of x working units as x->oo goes to zero? Empirical evidence about how hard people have found parallelization of factoring to be is just hand-waving.
Once you prove that SHA-256 encryption can't be broken without using brute force. You trying to draw this hard line of definitive proof shows that you don't have the wisdom to understand how the world works.